Barack Obama’s reputation among his fellow international players has deflated, his stature almost collapsed. In diplomatic circles, attitudes toward his leadership have been declining for some time, but this week you could hear the disappointment, and something more dangerous: the sense that he is no longer, perhaps, all that relevant. Part of this is due, obviously, to his handling of the Syria crisis. If you draw a line and it is crossed and then you dodge, deflect, disappear and call it diplomacy, the world will notice, and not think better of you. Some of it is connected to the historical moment America is in.

But some of it, surely, is just five years of Mr. Obama. World leaders do not understand what his higher strategic aims are, have doubts about his seriousness and judgment, and read him as unsure and covering up his unsureness with ringing words.

A scorching assessment of the president as foreign-policy actor came from a former senior U.S. diplomat, a low-key and sophisticated man who spent the week at many U.N.-related functions. “World leaders are very negative about Obama,” he said. They are “disappointed, feeling he’s not really in charge. . . . The Western Europeans don’t pay that much attention to him anymore.”

The diplomat was one of more than a dozen U.S. foreign-policy hands who met this week with the new president of Iran, Hassan Rouhani. What did he think of the American president? “He didn’t mention Obama, not once,” said the former envoy, who added: “We have to accept the fact that the president is rather insignificant at the moment, and rely on our diplomats.”