American presidents in recent decades have spent a great deal of time proclaiming U.S. leadership of the global system. The decision to withdraw from the Paris climate agreement undermines much of what they have said. For any student of global politics, it represents a watershed moment when it comes to debating America’s role in the world.

Becoming a global leader

In practice, global leadership can take two forms.

The first version confers leadership because a country is the most powerful. It has the strongest military, the biggest economy, the most innovative technology. But beyond that, a global leader has to be willing to cast aside its own short-term interests in favor of a longer-term outlook. This isn’t altruism. It is seeing beyond the horizon, what psychologists define as “enlightened self-interest.”

Sometimes a dominant power must endure costs to achieve a collective benefit. American behavior since 1945 has often fitted that description, from supporting NATO to setting up international institutions like the World Bank or funding others like the United Nations. It is why Americans describe themselves, in the words of former Secretary of State Madeleine Albright, as “the world’s indispensable nation.”

The second version confers leadership as a result of a country’s glowing reputation. Ronald Reagan, for example, invoked the biblical notion that the United States was a model, a “shining light upon a hill.”

America is not an ordinary country. It is special. And presidents since Reagan have routinely made that claim, as if it is a statement of fact. Sometimes global public opinion disagrees. But many Americans still see the country as “one stand[ing] above all others.”

Losing global leadership

It takes a long time and much effort to build a position as a global leader. But it can be lost in two ways.

The first is dramatic: through the heavy costs of war and with it the collapse of a leader’s economy. A country doesn’t even have to lose a war. Military victory in World War II cost Britain so much that it accelerated its decline as the preeminent power.

The second is a more incremental – and insidious. It involves a steady shift to a focus on short-term interests, and bullying – rather than cooperating with – other countries. This can take many forms. In economic terms, it may entail the leader imposing protectionist trade barriers against other countries. In security, the leader may require other countries to pay more for their collective defense. With that kind of behavior comes a loss of reputation. Sometimes leadership disappears with a whimper more than a bang. Individual decisions may seem so inconsequential that it is hard to spot their importance at the time.

Britain’s global leadership, for example, was built on free trade. But its first protectionist measures were introduced in the opening years of the 20th century. It took only another four decades for America to overtake it as the world’s economic leader. Interestingly, Ronald Reagan first employed protectionist trade barriers against Japan four decades ago.

Renouncing the Paris accord

Sometimes you can spot the moment that leadership was lost as clear as day. Withdrawal from the Paris climate accord looks like one of those moments.

Certainly, the Trump administration’s bickering over NATO’s budget has looked unseemly at times. But, despite Angela Merkel’s plaintive comment that it may be time for Europe to learn to be more self-reliant, that relationship is arguably reparable, contingent on some improvements in the defense expenditures of America’s NATO partners. It could simply be interpreted as tough American posturing.

The same is true of numerous trade deals. Indeed, senior Trump administration officials H.R. McMaster and Gary Cohn suggested in a Wall Street Journal op-ed only this week that “America First” did not mean “America alone” when it comes to both trade and national security.

But pulling out of the world’s most far-reaching global climate compact, signed by nearly 200 countries in the last two years, is a completely different animal. Renouncing the agreement makes the United States members of very a select group – with Nicaragua and Syria. And, unlike the U.S., neither of them is particularly important when it comes to greenhouse gas emissions.

Donald Trump may come to discover, like with so many other things, that extricating the U.S. from the Paris Agreement and negotiating a new one is more complicated than he imagined. But despite the fact that it could take years to resolve, it is impossible for the U.S. to retain the claim of leadership when it played an instrumental role in setting up the agreement - and then immediately defected from it.

Clearly, the Obama administration sold the climate deal to the American people as being in America’s self-interest. And numerous public interest groups eagerly reinforced that view. But it also served a much broader collective interest – to all of humanity.

That has been widely understood. In the last few days, even some of America’s foremost corporations lobbied in favor of continued American adherence to the agreement. This included Exxon Mobil, the world’s largest oil company.

So why withdraw? Well, adhering to the agreement, argued some pro-business think tanks, entailed domestic costs, particularly to workers employed in America’s fossil fuel industry. They are among Trump’s core supporters. The president’s rhetoric about the unfair nature of the deal aside, it is concerns about his core political constituency that has apparently proved paramount, at a cost to America’s global reputation.

The costs and benefits of withdrawal

There are likely few benefits to the United States. American coal is not in demand and the growth of the renewable power suggests that the demand for fossil fuels will gradually decline anyway

Certainly, the Trump administration can claim the reassertion of American sovereignty. And the decision may further consolidate Trump’s political support among his true die-hard supporters. As Trump said in his announcement, withdrawal is further evidence of his keeping his campaign promises. His promise of renegotiation, in contrast, isn’t feasible, and looks disingenuous.

But despite Trump’s claim to the contrary, the diplomatic and economic costs will likely be significant. And the greatest cost will likely be to America’s global leadership. It is hard to retain the pretense that a country leads when America First entails ignoring the pleas of its closest allies and the United Nations’ leadership.

Just as with the Trump administration’s withdrawal from global and regional trade agreements, China has expressed a willingness to step in and fill the void, and become “a leader on climate change.”

Historians decades from now will no doubt debate the issue of if and when America abdicated from its role as “the indispensable nation.” But, looking back, many may well claim that June 1, 2017 was the day that America’s global leadership ended.

Simon Reich does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond the academic appointment above.