But U.S. officials said that the determination to send weapons had been made weeks ago and that the chemical weapons finding provided fresh justification to act.

As Syrian government ­forces, with the help of Hezbollah and Iranian militias, began to turn the war in Assad’s favor after rebel gains during the winter, Obama ordered officials in late April to begin planning what weaponry to send and how to deliver it.

That decision effectively ended the lengthy disagreement among those in the White House — primarily Obama’s political advisers — who argued that providing arms would be a slippery slope to greater involvement, military leaders who said it would be too risky and expensive, and State Department officials who insisted that Syria and the region would collapse in chaos if action were not taken, officials said.

Even after Thursday’s announcement, critics in Washington, rebel leaders and even some U.S. allies described the prospect of sending light arms and ammunition as disappointing.