When President Trump announced his intention to withdraw from the Paris Agreement on climate change, California Gov. Jerry Brown, one of the agreement’s staunchest defenders, went straight to the capital. Only it wasn’t Washington—it was Beijing. Once there, Brown met with Chinese president Xi Jinping in the ornate Great Hall of the People and discussed the possibility of linking California to China’s massive cap-and-trade program for carbon emissions. In both form and substance, Brown’s visit looked more like that of the leader of a country than an American state. And while the governor’s trip to China is not the first case of subnational involvement in foreign affairs, it highlights more clearly than most the pitfalls of devolving diplomacy below the national level.

Many states, and several cities, including New York, established offices to promote trade and investment abroad as early as the 1970s, and often sponsor trade-promotion trips to foreign countries. These limited dealings with foreign countries have provoked little controversy. But in recent years, subnational diplomatic activity has expanded far beyond trade, and now includes much more contentious political issues that pit subnational against national policy positions. Climate change is perhaps the best example: since at least the early 2000s, mayors and governors in the United States, Europe, Brazil and elsewhere have pushed for much more aggressive climate policies than their own national governments, and subnational leaders were widely credited for building support for the Paris Agreement itself.

Despite these achievements, the growing global ambitions of state and local leaders risk competing with, rather than complementing, traditional nation-state diplomacy. This is especially true for the United States, where subnational diplomacy has recently come into vogue as a counterweight to the Trump administration’s isolationism. Governors ranging from Virginia Democrat Terry McAuliffe to Arkansas Republican Asa Hutchinson have carried messages abroad, usually promoting free trade, that run at least gently against the White House line. But despite their best intentions, in the world of diplomacy mayoral and gubernatorial assurances are a poor replacement for presidential ones.

For American states, the problem is partly constitutional. In 2012, the Supreme Court ruled that states can’t pass laws that have even an “incidental or indirect effect” on foreign affairs, an area the constitution reserves mostly for the federal government. These legal limits mean recent subnational agreements are inherently weak: in order to pass constitutional muster, California’s climate accord with China, for instance, is explicitly nonbinding. The Congressional Research Service, meanwhile, recently cautioned that the Hawaii legislature’s statement of support for the Paris Agreement might raise constitutional issues. Other countries have raised similar concerns about the involvement of subnational actors in foreign affairs. A 2014 Canadian government report warned, for example, that existing “multilateral institutions, forums and agreements may be undermined” by uncoordinated subnational diplomacy on the global stage.

On a practical level, most subnational governments lack the expertise and resources needed to tackle complex international issues. Even for those that do, like California, implementation can be a real challenge. Despite hopes that California’s attempt to link its carbon market with Quebec’s would raise carbon prices in both regions, prices remain stuck near the floor of $13 per ton of carbon dioxide, and the effort has been hobbled by different regulations, trade barriers, and other cross-border challenges. Sacramento’s proposal to join China’s much larger, more complex, emissions-trading system is likely to encounter even higher regulatory hurdles that it can’t overcome without support from Washington. These barriers mean subnational actors are limited in their ability to address complex global challenges on their own—and it would be reckless to believe that they can.

Instead, mayors, governors and other local leaders need to continue to work through national governments to build support for global initiatives, rather than around them. Subnational diplomacy is most effective when it supplements, rather than substitutes for, concerted national policies. This means that even when faced with an isolationist administration, mayors and governors concerned about climate change and other global issues can’t simply side-step Washington. Rather, they should appeal to the Trump administration’s stated goal of increasing exports, and press the economic case for federal assistance in forging clean-energy partnerships abroad, leveraging U.S. technology and expertise in the process. The administration, for its part, should recognize the value that subnational leaders can add to diplomacy, and restore Obama-era efforts to directly engage local leaders in foreign affairs.

Devolving diplomacy to cities and states carries plenty of appeal, particularly when national capitals seem dysfunctional. But even in a globalized world, subnational diplomacy is no substitute for the real thing.

Scott Moore is a Giorgio Ruffolo post-doctoral research fellow with the Belfer Center for Science and International Affairs and the Sustainability Science Program at Harvard University, where he studies intergovernmental environmental cooperation and subnational politics.