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When President Trump announced his decision to withdraw from the Paris Agreement, the backlash was clear and swift: two members of his advisory council quit; Obama issued an uncharacteristically harsh response; and leaders around the world expressed  regret. But most significantly, Trump’s unpopular decision sparked a growing grassroots movement among U.S. cities, states and businesses determined to reverse it.

In an open letter titled “We Are Still In,” an alliance of over 1000 governors, mayors, businesses, investors, and universities pledged to uphold the Paris Agreement in what they’re calling “the absence of leadership from Washington.” Over 200 “Climate Mayors” will adopt the agreement, while the governors of New York, California and Washington have created the bipartisan United States Climate Alliance, a coalition of U.S. states committed to doing the same. Trump may be pulling out of the landmark climate agreement, but Americans across the country are staying in it.

Such defiance is not only a symptom of Americans’ growing dissatisfaction with the Trump administration; it’s also a sign that the president isn’t the only one dictating America’s stance on climate change. From tech giants to local officials, powerful subnational groups and individuals are redefining America’s position on the Paris Accord. With their words and actions playing out in the international arena, outspoken city mayors are becoming pivotal actors in American foreign policy.

“I was elected by voters of Pittsburgh, not Paris,” Trump said, justifying his decision to withdraw from the accord. But the Mayor of Pittsburgh, Bill Peduto, responded with a tweet saying his city would comply with international pact. So who’s really defining America’s foreign policy?

As the Mayor of Pittsburgh, I can assure you that we will follow the guidelines of the Paris Agreement for our people, our economy & future. https://t.co/3znXGTcd8C

— bill peduto (@billpeduto) June 1, 2017

Local players have already begun to steal Trump’s limelight on the world stage. Less than a week after the president’s announcement, Mayor Peduto and his Parisian counterpart Mayor Ann Hidalgo co-authored an op-ed on the importance of global unity against climate change. Former New York mayor and U.N. special envoy for cities and climate Michael Bloomberg made a surprise visit to Paris to let the world “know the U.S. will meet our Paris commitment.” There, he met with French President Emmanuel Macron as if he were a stand-in for Trump.

The work of local players on global climate policy forces us to question not only who speaks on behalf of U.S. foreign policy, but also who enforces it. Beyond the rhetoric and symbolism, the leaders of America’s cities and states believe they can cut carbon emissions by the terms agreed upon in Paris. They’ll soon submit a plan to the United Nations outlining how.

Whether or not these alliances will be able to knock the country’s 2005 emission levels down by 26 to 28 percent is yet to be seen, but there’s reason to believe it’s possible. Globally, cities account for over 70% of all carbon emissions, and America’s cities certainly have the resources to do something about it. The states that make up the United States Climate Alliance comprise over a third of the national GDP. Meanwhile, Bloomberg has pledged $15 million to the U.N. agency responsible for implementing the agreement. Perhaps the former mayor is right: “Americans don’t need Washington to meet our Paris commitment.”

This isn’t the first time during Trump’s presidency when cities have sought to counter the administration’s policies. We saw it back in January when the mayors of “sanctuary cities” said they would defy Trump’s travel ban. And powerful interest groups have long used their power to influence government positions. But the current movement feels distinct: these subnational groups are not just lobbying the government or ignoring federal policies; they are actively working to bypass Washington in pursuit of foreign policy goals.

We have yet to see how successful their efforts will be. Local coalitions and wealthy individuals may aspire to change the course of American foreign policy, but they are up against a resilient state apparatus. And depending on what these alliances do moving forward, their actions may be considered unconstitutional.

Still, the acts of defiance of local and state leaders against Trump’s climate change decisions are more than just good news for environmentalists; they show us that cities, networks and businesses have the resources and the gumption to undermine the federal government—and make some foreign policy decisions of their own.