President Barack Obama’s environmental legacy will be remembered for its series of firsts. He is the first president to limit carbon pollution from power plants, curb methane emissions, and broker a climate deal with China. But none of these achievements ever captured the public’s imagination like the Keystone XL pipeline. 

Obama secured his reputation as an environmentalist on Friday, announcing that his administration has officially rejected the Keystone XL pipeline.

Obama’s decision to deny the pipeline its permit is his most significant, if symbolic, move to limit the growth of the world’s fossil fuel supply. His other climate initiatives have targeted how we consume fossil fuels, but he’s rarely intervened in the industry’s plans to extract and burn coal, oil, and gas in the first place. The Keystone refusal is the kind of declarative statement environmentalists have long wanted from a world leader, with Obama delivering a message that it’s finally time to keep fossil fuels in the ground. And the announcement comes mere weeks before officials gather in Paris to reach a global agreement that finally curbs greenhouse gasses. With Keystone decided, the United States has one more powerful example that it is making amends for its role as the biggest polluter in history. 

Keystone XL had an inconspicuous start, considering its role today as a bellwether in environmental politics. Two months before Obama won the 2008 presidential election, the company TransCanada submitted an application for a presidential permit to build the 1,179-mile long pipeline, which could carry up to 830,000 barrels of crude oil per day at peak capacity. To build the northern portion of the pipeline, which would run from Hardisty, Alberta, to Steele City, Nebraska, TransCanada required the State Department’s approval to cross the border (the already completed southern leg, now called the Gulf Coast pipeline, from Cushing, Oklahoma, to Texas’s coast, began operation in 2014).

The real controversy was over the kind of oil Keystone would be carrying. The tar sands in Alberta, Canada, contain a heavy crude oil, or bitumen, mixed in with sand, clay, and water. Compared to conventional oil, it requires extra energy to mine, dilute, and move, adding to its already hefty greenhouse gas footprint. The oil moved through the Keystone pipeline alone would be the equivalent of adding as much as 27 million metric tons of carbon dioxide each year according to a State Department environmental assessment, or 5.5 million cars on the road. 

When global oil prices begin to climb again, Canada's oil industry stands to grow massively in the coming years. Alberta’s tar sands contain an estimated 1.7 trillion barrels of oil, if the oil industry figures out how to extract every last drop—and technological advances in mining make it increasingly likely that it will. Alberta’s oil production first passed the one million barrel per day mark in 2004 and nearly doubled to 1.8 million in 2013. By 2020, Alberta’s government predicts that production will grow to 3.7 million barrels a day. Oil and gas companies need new pipelines to move all this oil, which is what makes Keystone so critical.

American environmentalists had little control over the development and decisions made by the more oil-friendly government in Canada. But pipelines needing Obama’s sign-off they could fight. Similar pipelines would normally get rubberstamp approval from the State Department. While Keystone was mired in the bureaucratic delay, the White House approved the Canadian company Enbridge’s application for another cross-border pipeline, the Alberta Clipper, in 2009. Keystone, however, attracted grassroots opposition, as Obama, who had been relatively silent on climate change, prepared for his reelection in 2012. Politically, Keystone became a lightning rod, leading the GOP-controlled Congress to pass a bill to try to force the administration to issue a final decision. The president vetoed it. 

Over seven years, Keystone has hit more than a few snags in the states, because the original proposed route cut through the fragile sand dunes of Nebraska’s Sand Hills and a giant aquifer. The company filed a work-around by 2012. Two years later, the State Department issued an environmental assessment that controversially found that Keystone “does not significantly exacerbate the climate problem.” Obama had promised to approve the pipeline only if it wouldn’t “significantly exacerbate” climate change. At this point, TransCanada waited only on Obama’s word, though the administration cited continued litigation in Nebraska as justification for the indefinite delay. 

There was a time when Keystone seemed inevitable. In a 2013 New Yorker article, Ryan Lizza noted that Obama in private argued that Keystone wasn’t an important part of his legacy; the EPA was doing the important business of fighting climate change.

In the last year he began to talk about Keystone differently. In public, he began taking a harsher tone as he ramped up his efforts on climate change. “Understand what this project is: It is providing the ability of Canada to pump their oil, send it through our land, down to the Gulf, where it will be sold everywhere else. It doesn't have an impact on U.S. gas prices,” he said in November 2014. In the spring, Obama told South Carolina students that the tar sands represent an “extraordinarily dirty way of extracting oil.” He hinted, “We're not going to authorize a pipeline that benefits largely a foreign company if it can't be shown that it is safe and if it can't be shown that overall it would not contribute to climate change.”

Sensing that the administration would reject Keystone,  TransCanada requested that the State Department delay its decision at least another year, which would have put the final decision in a new administration’s hands. The White House made clear this week that it wouldn’t honor TransCanada’s request.

Fighting climate change is an issue of cutting back on both the supply and demand of fossil fuels. And throughout Obama’s two terms, he has done a better job focusing on the latter. His achievements—promoting energy efficiency, fuel economy standards for cars, and the transition from coal to renewables in the power sector—all tackle the demand side, in an attempt to reduce America’s carbon addiction.

He’s often ignored the more difficult issue: the supply. He’s even enabled it. Obama approved Shell's plans to explore for more oil off Alaska’s coast in the Arctic this summer, proposed opening the Atlantic to offshore drilling, and leased public lands for coal mining. Keystone is a small part of the supply equation.

As Obama weighed his decision on Keystone, TransCanada and the rest of the industry have pursued alternatives, including expanding shipments by rail and tanker, filing an application for other large pipelines like Energy East in Canada, and even considering reapplying for a Keystone permit in the next administration.

Meanwhile, the environmental movement has shifted attention to other supply issues. Beyond blocking other proposed pipeline projects from Enbridge and TransCanada, they have targeted Arctic drilling, congressional efforts to lift the U.S. oil export ban, and the administration’s efforts to lease public lands for coal mining. 

From a scientific standpoint, Keystone is no more important than any of these other issues. But it was always more important for its symbolism.

Refusing Keystone because of its climate impact makes it more believable that we’ve reached a turning point on tolerating unlimited extraction and development of fossil fuels.

By taking a stand against Keystone, Obama has bolstered his weakest spot on climate change. Environmentalists are hoping that this new outlook doesn’t begin and end with the Keystone decision.