• “Big price to pay.”

An apparent chemical weapons attack that killed dozens of Syrians and sickened hundreds in Douma, the rebel-held Damascus suburb, increased international tensions and raised the possibility of a U.S. missile strike. Above, a video image of toddlers being treated.

President Trump, on Twitter, directly blamed both President Vladimir Putin of Russia and Iran for backing the Syrian regime of “Animal Assad.” U.S. security officials said a strike had not been ruled out. Russia and Iran claimed the reported attack was bogus.

The attack — and Mr. Trump’s hint at retaliation — came days after he pledged to quickly withdraw U.S. troops from Syria, highlighting the political and military risks of a war Mr. Trump has long wanted to leave.