President Obama boxed himself into a corner when he said last year that Syria’s use of chemical weapons against its people would be crossing a “red line” that could prompt a direct American response. Now that the United States has concluded with “high confidence” that President Bashar al-Assad has crossed that line, Mr. Obama had to follow through on his threat. As The Times reported, the administration has decided to begin supplying the rebels with small arms and ammunition.

That step would be a modest escalation of American involvement in response to the use of the nerve agent sarin, which was confirmed by the American intelligence. The White House said there were multiple incidents over the past year that caused an estimated 100 to 150 deaths. But that is a tiny fraction of the 93,000 Syrians believed killed in the civil war, now in its third year.

Nevertheless, Mr. Obama’s decision is highly significant because it opens the door to an even larger American role. Anti-tank weapons are also a real possibility. The White House said the chemical weapons finding had changed Mr. Obama’s calculus about the war but had not explained what that meant.

Mr. Obama has demonstrated a prudent reluctance to intervene directly in Syria’s civil war. Can the United States arm the rebels and avoid becoming enmeshed in another Middle East war? How will the administration keep weapons out of the hands of rebels who are affiliated with Al Qaeda and other jihadi groups? Does Mr. Obama believe this move will persuade Russia to stop arming Syria — or might it provoke President Vladimir Putin to ship even more supplies?