LONDON — Of the chemical weapons attack in Syria last week that left several hundred people dead, President Obama has now said: “We have concluded that the Syrian government in fact carried these out. And if that’s so, then there need to be international consequences.”

There are two presidential voices in that statement on PBS’s NewsHour, the active and declarative of the first sentence, the passive and impersonal of the second. They capture Obama’s oscillating drift over the Syrian conflict, now well into its third year, with more than 100,000 people killed and several million displaced. The president has had a bad Syrian war.

This is still an American-led and American-protected world. If “there need to be international consequences,” then the United States, in coordination with its allies and where possible with the backing of the United Nations, must deliver them. Obama has drawn and redrawn a red line at the use in Syria of chemical weapons, a scourge that almost all the world’s nations (189 of them) have abjured through the Chemical Weapons Convention and through participation in the Organization for the Prohibition of Chemical Weapons.

The credibility of the United States is a precious, already eroded commodity. Its loss would make for a treacherous world. That credibility cannot be compromised in this instance. A world where President Bashar al-Assad thumbs his nose at the U.S. president and where the international taboo on the use of chemical weapons lies shattered is headed in a very dangerous direction.