When nearly 400 millennials committed civil disobedience on Sunday in front of the White House to protest the Keystone XL pipeline, they sent a clear message to President Obama: “Stop this pipeline or the people will.”

Hailing from 80 universities and representing 43 states, over 1,000 youth climate activists converged on Washington, D.C., for a day of action called XL Dissent. They began by marching from Georgetown University, where President Obama gave his landmark speech on climate change last June, and then continued past Secretary of State John Kerry’s home, where marchers staged a mock oil spill, and finally reached the gates of the White House. Once there, hundreds of young people zip-tied themselves to the fence, imploring President Obama not to “lock” them into a dirty energy future by approving the Keystone XL. According to police, 372 people were arrested, but 350.org put the number at 398 — noting also that the current concentration of carbon dioxide in the atmosphere is 398 parts per million.

The historic XL Dissent action was notable for being both the largest single day of climate-related civil disobedience in U.S. history and the culmination of an unprecedented youth-led, organized and executed climate justice campaign. Far more than just a controversial tar sands pipeline, the battle over the Keystone XL is proving to be a flashpoint for political action among a generation whose future is imperiled by the continued combustion of fossil fuels.

“This isn’t merely Obama’s legacy,” said 21-year-old Tufts junior Evan Bell. “This is our lives.”

Like so many millennials, Bell sees climate change not as an abstraction, but as a reality that is already impacting millions and shows no signs of abating. In an XL Dissent press release published by The Nation, organizers were similarly earnest, writing, “We are young, awaiting a future fraught with uncertainty. This will not deter us from participating in an act of civil disobedience. Indeed, it has compelled us to organize one.”

Despite its historic outcome, XL Dissent evolved from humble beginnings. Last September, after a summer of organizing as a Fossil Free Fellow with the growing Fossil Fuel Divestment movement, 20-year-old Michael Greenberg of Columbia University formed a core team with Bell, 20-year-old Aly Johnson-Kurts of Smith College, 23-year old Nick Stracco of Tulane University, and others from across the country. Broadening their focus beyond campus divestment — a cause that funneled hundreds of participants into XL Dissent — they aimed to both radicalize the youth climate movement and expand what they called “the tar sands narrative.”

Organizers spoke frequently with several members of Tar Sands Blockade and citizens of frontline communities who fight the environmental injustices of carbon extraction on a daily basis. They also aimed to emphasize that the XL Dissent campaign is just one prong in the broader movement to stop the expansion of all fossil fuel infrastructure. As students chanted Sunday, “No more coal. No more oil. Keep your carbon in the soil!”

In order to transform their vision into reality, the organizers of XL Dissent combined social media savvy with operational flexibility. Despite receiving no institutional help in the beginning, the core team established a “loose organization,” according to Bell, that allowed people to take ownership of logistical priorities. This structure also allowed students the ability to offer whatever amount of time they could spare. More experienced activists stepped back as newcomers stepped forward.

When the State Department issued its recent Environmental Impact Statement for the Keystone XL — derided by NASA’s recently retired chief climatologist James Hansen as “pure scientific garbage” — interest in XL Dissent skyrocketed. The historic youth-planned action soon attracted institutional support from 350.org, DC Action Lab and the Energy Action Coalition among others.

Despite targeting President Obama, many of the organizers credit him with galvanizing their activism. Bell suggested that the millennials’ strong “generational connection” to President Obama and the hope he cultivated mobilized hundreds to take action against the pipeline and, ironically, the president himself.

“We asked him to live up to his ideals,” Bell explained. “But he hasn’t. So we are.”

Obama’s waffling on the Keystone XL not only promises to motivate millennials, but also to alienate a core demographic. According to Stracco, “If the Democratic Party wants to keep our vote, they better make sure President Obama rejects that pipeline. We voted for a climate champion, not a pipeline president.”

A recent poll conducted by the League of Conservation Voters echoes Stracco’s sentiments, confirming that 73 percent of young voters would vote against a president who did not take action on climate change. Meanwhile, 80 percent want Obama to take action now.

As Jamie Henn, co-founder of 350.org recently argued, “If Democrats were smart, they’d be more focused on keeping these students out of jail and getting them into voting booths. That’s going to take a presidential decision to reject the Keystone XL.”

Prominent climate activist Bill McKibben lauded XL Dissent as possibly “the biggest single day of civil disobedience in the whole Keystone saga.”

No matter the fate of the Keystone XL, it is clear that millennials are just beginning to assert their agency. As Hampshire College student Noga Heyman declared after the XL Dissent action, “The fight’s not over and neither is our escalation.”