On July 17, California’s State Assembly and Senate voted to expand and extend through 2030 the state’s pioneering cap-and-trade program to curb greenhouse-gas emissions. It was a huge victory for Governor Jerry Brown, who had insisted that both houses approve the measure by a two-thirds majority to protect it from legal challenges.

Brown’s triumph, which he signed into law on July 25, reinforced California’s status as the United States’ leader in beginning the transition to a post-carbon economy. The governor has insisted that far-sighted leadership in Sacramento can fill the void created by President Donald Trump’s disastrous withdrawal from the Paris climate agreement.

But can the wealthiest and most populous U.S. state conduct its own foreign policy within the U.S. constitutional system? Writing in Foreign Affairs in 1993, the journalist James O. Goldsborough pondered whether California should do so. A quarter of a century later, the verdict is in. As the United States has receded from global climate leadership, California has filled the diplomatic breach.

ALL THE WORLD'S ITS STAGE

Since last year’s presidential election, California has emerged as a focal point for resistance to the Trump administration’s policies. This is hardly surprising. Trump lost the state by a wide margin, garnering less than one-third of the popular vote in the general election, and his opponent, Hillary Clinton, bested him by 30 percentage points—the largest victory for a Democrat in the state since 1936. And yet on election night, California’s progressive voters discovered just how far removed they were from the national mood. Trump won the electoral college after running on a nativist, protectionist, and anti-science platform at odds with most of California’s ethnic diversity, commitment to open trade, and embrace of environmentalism. The stage was set for confrontation.

Solar panels on the roof of a building in Los Angeles, June 2017. MIKE BLAKE / REUTERS

Since then, Brown has staked out progressive positions starkly at odds with the White House’s views on immigration, trade, and climate change. On January 24, four days after Trump’s dark inaugural address in Washington, Brown shot back in his own defiant “State of the State” speech in Sacramento. “California is not turning back,” he declared. “Not now, not ever.” If the United States was no longer willing to serve as an example to the world, Brown argued, his state would rise to the challenge. “We must prepare for uncertain times and reaffirm the basic principles that have made California the Great Exception that it is,” he said.

Brown pledged that his state would remain open to immigrants (some 27 percent of California residents were born abroad). He also promised that his state would speak truth to power on climate change. “Whatever they do in Washington, they can’t change the facts,” Brown said. “The science is clear. The danger is real.” California would work with “other states and provinces and even countries to stop the dangerous rise in climate pollution,” he said.

But what, really, could California do? The U.S. Constitution gives the federal government responsibility for foreign and defense policy. Articles I and II apportion powers to the executive and legislative branches to conduct diplomacy, negotiate and ratify treaties, provide for the national defense, declare war, regulate foreign trade, and control borders and immigration, among other functions.

California’s status as a subnational actor makes it easier for it to partner with China.

To be sure, the 50 U.S. states retain considerable rights under the Tenth Amendment, which declares that those powers that the Constitution does not explicitly confer to the federal government are left to the states and to the people. These so-called reserved powers include the right to hold elections for state and local governments, manage public schools, and regulate business within the state. As the Supreme Court explained in the 1995 case U.S. Term Limits, Inc. v. Thornton. “The Framers split the atom of sovereignty. It was the genius of their idea that our citizens should have two political capacities, one state and one federal, each protected from incursions by the other.”

Of course, the precise boundaries between the authorities of the federal and state governments have been contested throughout U.S. history—most dramatically, during the Civil War. Since that conflict, conservatives have been most insistent on the inherent right of states to nullify objectionable federal legislation. And yet typically, states’ efforts to exercise their residual sovereign powers have stopped at the water’s edge: dealings with foreign governments and global challenges have remained the domain of the federal government.

THE GREEN AND GOLDEN STATE

This history makes the standoff between California and the Trump administration especially striking. Not only is a progressive governor flexing his state’s muscles against the federal government; he is doing so on matters of global importance, in concert with foreign officials. As Brown has sought to use California’s massive economic weight as the world’s sixth-largest economy to spur more effective climate governance, he has received a hearing from foreign leaders unsettled by the Trump administration.

For Brown, this is familiar terrain. In 2013, the governor signed an agreement with Quebec to link California’s carbon emissions with that of the Canadian province—a measure meant to be a first step in the development of a North American carbon market. Two years later, before the Paris climate conference, Brown joined with the minister-president of the German state of Baden-Wurttemberg to establish the Under2Coalition, an international alliance of cities, states, and nations committed to limiting the average increase in global temperature to less than two degrees Celsius above pre-industrial levels. By the spring of this year, the coalition included 170 jurisdictions across six continents, encompassing 16 percent of the world’s population and 39 percent of its GDP. The group’s members pledged to reduce their greenhouse-gas emissions by between 80 and 95 percent below 1990 levels by 2050.

Brown redoubled his efforts after June 1, when Trump withdrew the United States from the Paris Agreement, the most significant international agreement of this century. Hours after the announcement of the United States’ pullout, Brown, New York Governor Andrew Cuomo, and Washington Governor Jay Inslee announced the creation of what they called the U.S. Climate Alliance, a coalition dedicated to upholding the Paris agreement despite Trump’s departure from it. California’s two Democratic senators, Diane Feinstein and Kamala Harris, joined the governor in denouncing Trump’s decision.

U.S. President Donald Trump after announcing the United States' withdrawal from the Paris agreement, Washington, June 2017. KEVIN LAMARQUE / REUTERS

The next day, Brown left for a week-long trip to China to underscore California’s commitment to working with that nation on clean-energy issues. In Beijing, he met with Chinese President Xi Jinping, declaring that California would stay the course. Brown toured China for several days, comparing notes on carbon emissions reduction efforts with Chinese officials, signing agreements to develop clean-energy technology with provincial governments, and touting his state’s five-year-old cap-and-trade market—a version of which China hopes to create by the end of this year. The governor’s underlying message, amplified by the state’s strong recovery from the Great Recession, was that ambitious climate goals are no barrier to strong economic growth.

Counterintuitively, California’s status as a subnational actor makes it easier for it to partner with China. Whereas the U.S.-Chinese diplomatic relationship involves a number of controversial issues—from currency manipulation to navigational rights in the South China Sea—the relationship between California and Beijing is less sensitive. So although California cannot compete with the U.S. government as an independent, sovereign actor, it can help fill a policy vacuum, persuading China—and other foreign governments—that a “no” from Washington on climate-change cooperation need not be the entire story, much less the final word. This appears to be Brown’s own thinking, too. As his biographer, the China expert Orville Schell, told the Sacramento Bee, “California is now a real port-in-the-storm of the post-Paris agreement world of Trumpian climate denial in which we now must live.”

California has demonstrated its growing influence on the international stage.

Whether that remains the case depends in part on whether the Trump administration gives California free rein to continue pioneering U.S. climate policy or instead tries to clamp down on the state’s freelancing. The White House could attempt to constrain California by threatening to reduce federal spending there (as Interior Secretary Ryan Zinke recently did to Alaska after Senator Lisa Murkowski’s vote against the repeal and replacement of the Affordable Care Act). There have been some signs that the administration has considered doing this: in February, the president told Fox News that “California in many ways is out of control” and that he might need to “defund” the state unless it changed its policies (in this case, on undocumented immigrants). Already, the Trump administration has delayed a grant to support a high-speed rail line from San Francisco to Los Angeles; in the future, it could try to zero out the funding that Congress might attempt to grant that project in an infrastructure bill. The administration might also seek to hobble California’s ability to serve as the nation’s pacesetter on environmental policy.

One indication of whether Washington will make such moves could come in the regulatory field. Since Richard M. Nixon’s presidency, the Environmental Protection Agency has granted California a waiver to impose fuel-efficiency standards that are stricter than nationwide levels. The effect has been catalytic, for instance by driving big automakers to raise the efficiency of their fleets to gain access to the state’s massive market. Many environmentalists fear that Trump’s EPA will strike down this provision: if it does so, California’s clout in setting the nation’s regulatory standards will diminish.

In any case, however, California has demonstrated its growing influence on the international stage—and its capacity to fill a policy vacuum left by Washington. Even the world’s most powerful nation, it appears, can face competition from a determined subnational actor. And that is as it should be. In today’s world, diplomacy is too important to be left to sovereign states.