SAN JOSÉ, Costa Rica — I am not one of those who believe the United States’ days of world leadership are behind us. All my life I have been an admirer of our northern neighbor. I believe its strengths — its democracy, its founders’ wisdom, its people’s ingenuity and diversity — give it unique authority in the world. But many of its citizens and their leaders seem to take such authority for granted now, as if it were American property. The truth is, it must be earned, and today the United States is passing up opportunities to earn it.

The Arms Trade Treaty, approved by the United Nations in April, is one such opportunity, and it must not be allowed to slip by. Last month, Secretary of State John Kerry signed it. Given the enormous presence of the United States in the international arms market, the treaty’s ratification, which requires a vote of two-thirds of the Senate, is essential to its success.

But the treaty will face stiff opposition. Two Republican Senators, James Inhofe and Jerry Moran, have gleefully pronounced the treaty “dead on arrival” and argue that the United States should not ratify it because North Korea, Syria and Iran declined to sign it. To do otherwise, they assert, would leave the United States “handcuffed.”

Gentlemen, you have it backward. The United States would not be handcuffed by a treaty that prevents the sale of conventional weapons to individuals or states that would use them to violate human rights. If the Senate fails to ratify the treaty, your country will be handcuffed by its own reluctance to lead. The United States, which claims to desire a safer, more peaceful world, would shrink from moving toward that goal unless the rest of the world acted first.