Any new vision for American greatness in the world must flow from an understanding of how the country has changed since 2001. We are still one of the richest nations on earth, but Americans are poorer, war-weary and irritated with what appears to be the ingratitude of nations for which we have sacrificed a great deal in blood and treasure. There are substantial wings of both the Democratic and Republican parties that wish to wash their hands of the world’s troubles.

In that environment, Mr. Romney must give a clear explanation of how American power since the end of World War II provided the foundation for the most prosperous and successful era in human history; how our domination of the world’s most trafficked waterways has permitted the flourishing of trade; and how exporting our principles of political and economic freedom has opened and nourished markets that buy American goods, employ American workers and allow Americans to enjoy an unmatched level of security.

More important, Americans must know that it is not for mercantile benefits alone that the United States has exerted its leadership. It is because there is no other power, and no other people, that can — or, if able, would — exert the benign influence that has characterized our role in the world. Whether you like the Iraq war or hate it; like the battle in Afghanistan or not; believe in the ouster of Col. Muammar el-Qaddafi or revile it — in no case has the United States intervened for malevolent purposes.

Unfortunately, Mr. Romney hasn’t made that claim. Instead, when asked for specifics, he has outlined an Iran policy that doesn’t differ markedly from Mr. Obama’s. When pressed on what he would do differently in Syria, he has trodden so carefully that he has found himself to the left of his party’s internationalist wing. And he has doubled down on the notion that Russia remains a geostrategic threat, without presenting any persuasive evidence that it is.

It’s not that Mr. Romney does not or cannot offer a more compelling vision of American leadership. Having heard him speak privately, and having met him on a few occasions, I believe he has one. Now is the moment to show it.