WASHINGTON — With rebel forces in Syria in retreat and the Obama administration’s policy toward the war-ravaged country in disarray, Secretary of State John Kerry arrived at the White House Situation Room one day in June with a document bearing a warning. President Bashar al-Assad of Syria had used chemical weapons against his people, the document said, and if the United States did not “impose consequences,” Mr. Assad would see it as a “green light for continued CW use.”