VENTURA, Calif. — The offices of Patagonia occupy a low-slung complex of stucco buildings in this sleepy beachside town in Southern California. There are solar panels and picnic tables in the parking lot, day care with a jungle gym by the main lobby and easy access to the beach, where employees surf during lunch break. It is a corporate Eden of sorts, where idealistic Californians run a privately held company that sells about $1 billion of puffy down jackets and organic cotton jeans each year.

But on an unseasonably hot and windy Monday morning in early December, Patagonia headquarters were transformed into something that quickly resembled a war room. There were emergency conference calls with Washington lawyers. Court filings were prepared. Web designers remade the company’s home page.

It wasn’t a business crisis that had mobilized the company, however. It was politics.

Hours earlier, President Trump had announced plans to sharply reduce the size of two national monuments in Utah. Bears Ears, an expanse of red-rock canyons rich with archaeologically significant sites, would be slashed in size by 85 percent, more than one million acres. Another monument, Grand Staircase-Escalante, would be reduced by half.

Mr. Trump said the decision was about reducing federal overreach. “Some people think that the natural resources of Utah should be controlled by a small handful of very distant bureaucrats located in Washington,” he said. “And guess what. They’re wrong.”

Yet to the tribal groups and conservationists who had been monitoring the situation, including Patagonia, the decision realized some of their worst fears: that the Trump administration would be waging an assault on public lands, and potentially opening up protected areas to drilling and mining.

Patagonia was as ready for this moment as any company could be. For more than 45 years, the company has mixed business and politics to a degree unusual in corporate America. While companies are expected to weigh in on everything from gun control to transgender rights these days — and many do so uncomfortably — Patagonia has been unapologetically political since the 1970s.

It bills itself “the Activist Company” and publicly advocates for environmental protection, fair trade and stricter labor standards. It supports thousands of grass-roots environmental activists, and has been involved with Bears Ears since 2012. But until December, Patagonia had never tangled with a president.

That Monday morning, about 50 Patagonia employees gathered in a conference room to watch Mr. Trump’s speech. The mood was somber. Then, within an hour of the president’s remarks, Patagonia updated the home page of its website. Instead of promotional images advertising colorful products, there was a stark message against a black background: “The President Stole Your Land.”

At the same time, Patagonia’s legal team set into motion a plan that had been in the works for months: It would sue the president.

Working with a handful of local groups and the law firm Hogan Lovells, Patagonia filed a lawsuit in Federal District Court in Washington. The lawsuit named as defendants Mr. Trump, Interior Secretary Ryan Zinke, the secretary of agriculture, the director of the Bureau of Land Management and the chief of the Forest Service. And the argument was simple: The Antiquities Act of 1906 gave presidents the power to create national monuments. But it did not grant the power to reduce them.

“For as much authority as it gave to the president to create these monuments, Congress gave the president no authority to revoke or modify those monuments,” the lawsuit reads. “Congress is the sole authority that can undertake such changes.”

Patagonia’s activism has made the company plenty of enemies over the years. Developers, the fossil fuel industry and lawmakers have all gone after the company, and this time was no exception. As Patagonia ramped up its campaign, the Trump administration hit back. Mr. Zinke and Republicans in Congress accused Patagonia of playing politics to sell more clothes, and the hashtag #BoycottPatagonia began circulating on Twitter.

The barbs only stiffened Patagonia’s resolve. The company’s founder went on CNN and called the Trump administration “evil.” And Rose Marcario, Patagonia’s chief executive officer, made the protection of Bears Ears one of her top priorities.

“We’ve always given to grass-roots organizations; it’s part of our D.N.A.,” she said in a recent interview at company headquarters, an American flag flying outside her window. “But the fight got a lot more urgent after the election. This was totally unheard-of.”

It would have been enough for one day if Patagonia had just been caught up in a public spat with the president. But hours after Mr. Trump’s announcement, a more urgent crisis emerged: A brush fire was reported in Santa Paula, Calif., a small community not far from Patagonia’s headquarters.

The hot, dry Santa Ana winds blowing that day whipped the fire into an inferno, and much of Ventura was soon evacuated. At the very moment Patagonia was preparing to sue the president, its campus was shut down indefinitely and employees fled their homes.

“We did a lot of scenario planning for the lawsuit,” said Corley Kenna, Patagonia’s head of communications. “We didn’t put on the list that life-threatening fires would be happening at the same time.”

Protecting a River

Patagonia was founded by Yvon Chouinard, an enigmatic mountain climber with an interest in Zen Buddhism and a passion for the environment. In 1957, he taught himself to blacksmith, and began making and selling climbing gear that was less damaging to the rocks he and his buddies were scaling in Yosemite National Park.

Within a few years, he had set up shop in Ventura and was doing a brisk business selling clothing for outdoor enthusiasts. He eventually called the company Patagonia, a homage to the vast mountainous region at the southern tip of South America, where he had recently traveled.

Patagonia developed a cult following and expanded its offerings. But for the most part, the company was a means to an end, a source of funds that enabled Mr. Chouinard and his friends to surf, climb and travel their way around the world.

That changed in 1972, when Mr. Chouinard attended a City Council meeting to hear about development plans along the Ventura River. Under a proposal being considered, the flow of the river would have been changed, and a prized surf break could have been ruined.

At the meeting, it looked as if the development would proceed. Then a young environmental activist, Mark Capelli, took the floor. He presented a slide show and argued that the proposed changes would harm the birds, water snakes and muskrats in the estuary. The development was halted, the river was protected, and the surf break was preserved.

Mr. Chouinard befriended Mr. Capelli and began to support his work, giving free office space to his nascent organization, Friends of the Ventura River, and helping to fend off several more attempts to develop the river.

That set the template. Patagonia would offer small grants to local activists, give in-kind support through marketing know-how and business savvy, and amplify their message with customers. Mr. Chouniard also resolved to give one percent of Patagonia’s sales to support environmental activism.

Patagonia has given out thousands of grants. In Alaska, the company has supported efforts to prevent waste from mining operations from polluting Bristol Bay, a productive salmon fishery. In Yellowstone National Park, Patagonia has worked to protect grizzly bears. In Poland, the company has advocated for the protection of forests. And Patagonia has waded into national politics before, having opposed the North American Free Trade Agreement and the Trans-Pacific Partnership.

The company has produced feature-length documentary films, including “DamNation,” an argument against damming rivers. Every other year, it hosts a conference where activists share tips and best practices for protests, fund-raising and litigation. And in 2011, Patagonia famously placed an ad in The New York Times that read, “Don’t Buy This Jacket,” an effort to discourage excessive consumption.

“Patagonia has established a unique role in the political and policy ecosystem, and are willing to be very public about their advocacy,” said Neil Kornze, who was a director of the Bureau of Land Management under President Barack Obama and now runs an environmental consulting firm. “They are a group of avid environmentalists who just happen to sell coats.”

Business is booming under Ms. Marcario. A former private equity executive who walked away from finance to study Buddhist meditation in India for two years, Ms. Marcario joined Patagonia in 2008 as chief financial officer, drawn to the company’s environmental activism.

“That’s why I’m not on Wall Street right now making a ton of money,” she said. “I’m kind of worried about the planet.”

She was named chief executive in 2014, and has overseen Patagonia’s continued expansion. Revenues and profits have quadrupled over the past 10 years, according to the company.

Under Ms. Marcario’s guidance, Patagonia has also struck out in new directions. It started a venture capital arm, Tin Shed Ventures, named after the metal hut — still standing — where Mr. Chouinard once forged and hammered metal, and has invested some $75 million in eco-friendly start-ups.

It has opened a food business, Patagonia Provisions, selling dried buffalo, lentil soup and beer. It recently activated a social network of sorts, Patagonia Action Works, designed to connect engaged consumers with local environmental campaigns.

And the company maintains a team of 18 people focused on supporting activism and distributing grants. Since 1985, Patagonia has given away some $90 million to environmental causes.

“Science without activism is dead science,” Mr. Chouinard said in a recent interview. “We want to fund the little activist organizations that are out there on the front lines, the grandmothers in front of the bulldozers.”

Writing on the Wall

In 2012, one of the little activist organizations that Patagonia supported for the first time was Friends of Cedar Mesa, a small nonprofit organization advocating for the protection of a wide swath of sensitive desert in southern Utah, including the area that would become Bears Ears.

At the time, Republicans in Utah, notably Representative Rob Bishop, were intensifying efforts to reduce federal protections of some lands in the state. Doing so could open the door for more oil and gas drilling and uranium mining, bringing in additional state tax revenue.

Patagonia ramped up its support for Friends of Cedar Mesa, offering grants and producing a short film about Josh Ewing, the organization’s director, who goes rock climbing in the area. It was the kind of assistance Patagonia had offered to thousands of local groups over the years, and at the time, there was no sense that Bears Ears would soon become a point of contention in a national debate over public lands.

“Patagonia has been interested in this area for a long time,” Mr. Ewing said. “More than any company I’ve ever seen, they put their money where their mouth is.”

As Patagonia learned more about Bears Ears, which was a favorite of rock climbers, it came to appreciate the area’s significance. Sacred land to tribal groups and home to Hopi and Navajo ruins and petroglyphs carved into rock walls, it was a largely unspoiled terrain that could be threatened by expanded drilling and mining.

“As we got more involved, we recognized the cultural and spiritual importance of the place,” said Lisa Pike Sheehy, Patagonia’s vice president of environmental activism. “This was a place that should have been protected 50 years ago.”

In total, Patagonia has spent some $2 million on efforts to protect Bears Ears, including making grants to nonprofit groups, producing films and other marketing materials, and buying television ads. And at least for a brief moment, they worked.

In December 2016, with less than a month left in his administration, Mr. Obama designated two new national monuments, including Bears Ears. It was a last-ditch effort by Mr. Obama to secure as much of his environmental legacy as possible. But with Mr. Trump’s inauguration looming, supporters of the monuments knew the victory might be fleeting.

“We never got the chance to celebrate the monument being created,” Mr. Ewing said. “The writing was on the wall. There wasn’t ever a moment when we thought the monument was safe.”

Inside Patagonia, Ms. Marcario and her staff knew their resolve would soon be tested. On the morning after Mr. Trump’s election, Ms. Marcario sent out an all-staff email. “Defending our air, soil and water has never been more important,” she wrote. “In the last few years, our voice and reach has grown louder and stronger … we won’t stop.”

The fears were quickly realized. Shortly after Mr. Trump took office, he ordered a review of some national monuments, vowing to “end another egregious use of government power.”

During a monthslong public comment period, the Interior Department received more than two million submissions, the majority of them supporting the continued protection of public lands. Patagonia said its customers had submitted more than 150,000 comments.

During the first months of the new administration, Ms. Marcario and another Patagonia employee, Hans Cole, made numerous attempts to meet with Mr. Zinke as part of outdoor industry delegations, according to internal Interior Department emails reviewed by The Times. No meeting with Mr. Zinke ever took place, but Mr. Cole joined two meetings with Interior Department staff.

Around this time, Patagonia also began planning for its eventual lawsuit. It enlisted Hogan Lovells, which tracked developments on the issue and organized a weekly conference call for the eventual plaintiffs.

Meanwhile, Patagonia ramped up its publicity efforts around Bears Ears and public lands. The company worked with Google to start a websitefeaturing videos, virtual reality and information about Bears Ears. It aired its first-ever television ad, featuring Mr. Chouinard sitting by a river, reflecting on the virtues of wild places.

And Patagonia succeeded in its monthslong, retaliatory campaign to move a lucrative trade show out of Utah. For years, the Outdoor Retailer trade show had taken place in Salt Lake City, bringing business and tourism revenue to the state. But when politicians didn’t budge after Patagonia demanded that they voice their support for public lands, the Outdoor Retailer show was moved to Denver.

The retailers added an economic argument to the campaign to protect Bears Ears: Outdoor recreation contributes more than $12 billion annually to the Utah economy, spending that could be jeopardized if public lands are developed.

In the end, none of this made much of a difference. In August, Mr. Zinke proposed reductions to four monuments, including Bears Ears. Then Mr. Trump made his proclamation on Monday, Dec. 4, and all hell broke loose.

‘Ash Falling Like Rain’

That evening, Ms. Marcario was driving north along the California coast, returning home from a business trip. As she approached Patagonia’s headquarters, she could see flames encircling Ventura.

“I saw the whole ridge line on fire,” she said. “I realized we wouldn’t be able to go back to the office.”

Patagonia enacted its emergency plan overnight. Employees were instructed to stay away from campus, and the company offered to pay for accommodations for anyone who had to evacuate.

A skeleton crew stayed behind at Patagonia, putting out spot fires as they flared up on campus. The fire spread, surrounding Ventura and consuming hospitals and apartment buildings on the outskirts of town.

“There was ash falling like rain,” Ms. Marcario said. “It was something I have never seen before.”

As the fire spread north, Patagonia and the Trump administration sparred publicly. Mr. Chouniard, typically a recluse, took to the airwaves to bash Mr. Trump. “This government is evil, and I’m not going to sit back and let evil win,” he said on CNN.

Mr. Zinke hit back. “You mean Patagonia made in China?” he said on a press call, accusing the company of playing politics in a gambit to sell more clothes. “I think it’s shameful and appalling that they would blatantly lie in order to gain money in their coffers.”

Then on Fox Business Network, Mr. Zinke took issue with Patagonia’s activism. “They should focus on how to bring manufacturing back to this country, rather than lying to the public about losing federal land,” he said.

The House Committee on Natural Resources, chaired by Mr. Bishop, joined the fracas. “Patagonia is lying to you,” the committee wrote on Twitter. “A corporate giant hijacking our public lands debate to sell more products to wealthy elitist urban dwellers from New York to San Francisco.”

Soon, the hashtag #BoycottPatagonia began circulating among Trump supporters on Twitter, and Stephen Colbert worked Patagonia into his “Late Show” routine.

As Patagonia and Trump supporters traded barbs, the blaze, known as the Thomas Fire, became the largest wildfire in modern California history. Burning for more than a month, it destroyed more than 1,000 structures and forced the evacuation of some 100,000 people. Downtown Ventura and the Patagonia campus were spared, but five employees lost their homes.

With the fire still burning out of control on Dec. 6, Ms. Marcario was working from her home in Montecito, 30 minutes north of headquarters. Friends and employees who had fled were staying with her, though Ms. Marcario would later be forced to evacuate, too.

“Then in the middle of what felt like the apocalypse, across my inbox comes an email,” she said. “Patagonia versus Donald Trump, et al.”

Ms. Marcario placed a call to her executive team and informed it that she had approved the lawsuit. Patagonia was suing the president.

‘The President Stole Your Land’

Since that week in December, the feud between Patagonia and the Trump administration has continued to simmer.

Mr. Bishop invited Mr. Chouinard to testify before the Committee on Natural Resources. Mr. Chouinard declined the offer, calling any hearing a “macabre celebration of the largest reduction in public lands in American history,” and the committee part of a “failed Orwellian government.”

Mr. Bishop replied to Mr. Chouinard with a tart letter of his own. “Although it is your right, living in a bubble isn’t healthy, nor is it conducive to a robust discussion on important matters of public policy,” he said. In an email, the committee’s press secretary said Mr. Chouinard was still welcome to testify.

Then in March, The Times published an article that showed oil and gas interests were central to the Trump administration’s decision to shrink Bears Ears. After that, Patagonia updated its website yet again. The new copy: “The President Stole Your Land and You Were Lied To.”

“The decision was nothing more than a political favor,” Ms. Sheehy, the company’s vice president of environmental activism, wrote in a blog post. “The redrawing of boundaries was deliberate, and directly influenced by an industry that spends millions of dollars lobbying the government to get what it wants.”

The Trump administration has not formally responded to the lawsuit filed by Patagonia and the other plaintiffs. Heather Swift, a spokeswoman for the Interior Department, said in an email that claims that the redrawing of boundaries was motivated by oil and gas interests were “patently and demonstrably false.”

Ms. Marcario draws a link between the extraction and consumption of fossil fuels and the Thomas Fire, which some climate scientists believe was made worse by climate change.

“These are climate disasters, and they’re going to get worse and worse,” Ms. Marcario said. “We have an administration that isn’t paying any attention to that.”

And the disasters weren’t over yet. In January, torrential rains hit the area. Hillsides eroded by the Thomas Fire came loose, and devastating mudslides roared through Montecito, killing more than 20 people and damaging the homes of several Patagonia employees.

Patagonia’s lawsuit against Mr. Trump is currently tied up in the courts. It has been consolidated with two other cases filed by separate groups, and the government has asked a judge in Washington to transfer the case to Utah. There is no deadline for that decision, and until then, all proceedings in the case are stayed. Hogan Lovells, which has 25 lawyers working the case on a pro bono basis, has billed $1.7 million in hours to date.

In the meantime, the area originally protected by Bears Ears National Monument is available for commercial use, including drilling and mining, though there hasn’t yet been any new development.

At Patagonia, Ms. Sheehy said that while the company would see its suit against the president through to the end, it had an expansive time horizon when it came to environmental protection.

“These are all long fights,” she said. “We’re in this for 50 years, 100 years.”