President Donald Trump’s “America First” foreign policy won’t work if multilateralism is relegated. With that in mind, Trump should recommit the United States to its traditional role of global leader as he addresses the United Nations General Assembly on Tuesday.

Trump criticized the U.N. and other international institutions, especially during the 2016 campaign. But encouragingly, Trump was more conciliatory in his brief comments at the U.N. on Monday, urging members to rein in costs and bureaucracy in order for the U.N. to reach its “full potential.” Those goals are shared by U.N. Secretary-General Antonio Guterres, who has proposed changes that have led U.S. Ambassador to the U.N. Nikki Haley to acknowledge that the U.N. is “totally moving toward reform.”

Trump should amplify Haley’s assessment when he has the world’s attention Tuesday. He is also expected to rail against North Korea’s unchecked weapons development and Iran’s destabilizing behavior in the Mideast. The president should add other rogue regimes, including Myanmar, which is executing what Guterres rightfully calls “ethnic cleansing” against the Muslim minority Rohingya.

On these and other issues an international response is not only warranted, but necessary in order to prevent a spiral into conflict that would inevitably involve the United States. The North Korean crisis, for instance, has been successfully cast by Kim Jong Un as a showdown with the U.S. when in reality every country — including and especially China, Pyongyang’s patron — has a stake in a negotiated outcome. The window for such a diplomatic option, however, is narrowing.

Other threats aren’t nearly as defined by national borders, however, and are more transnational in nature. Among them are the global migration crisis, in which the numbers of refugees, economic migrants and other displaced people are at a deeply destabilizing postwar high. Trump’s hostility to the plight of these people was an attractive factor to some of his core supporters during the campaign. But continuing it would greatly hurt his efforts to internationalize solutions to vexing problems and could counterproductively draw in the U.S. to the kinds of conflicts Trump has pledged to avoid.

Climate change is another borderless menace already exacerbating security concerns, a fact acknowledged by nearly every other nation as well as the Pentagon. Although it’s unlikely given his previous presidential proclamation, there is no bolder and better move Trump could take than to announce that the U.S. would stick with the Paris climate accord.

While substance is always most important in foreign policy, style counts, too. A statesmanlike approach to Tuesday’s address would likely make Trump’s prioritization of U.N. reform more successful. Conversely, if he repeats the kind of provocative presidential behavior displayed at this summer’s NATO summit it would limit the odds of a positive outcome.

For America’s — and the world’s — sake, we hope he chooses his words wisely.