Needing to relieve himself, the man stepped out of a bus into the night air of the Syrian desert.

The bus was part of a convoy carrying 300 Islamic State fighters that had been stuck there for days, prevented by American bombers from moving forward and prevented by the presence of women and children from being bombed.

The man walked a short distance away and began to urinate.

An American Hellfire missile ended the excursion.

By the American military’s count, 20 Islamic State fighters in the convoy died like that.

Those killings were among the consolation prizes the Americans claimed after ending the two-week standoff this week, reversing a vow never to let the militants pass and yielding a tactical victory to the Islamic State.

American officials say the decision to withdraw was the result of a complicated trade-off of competing priorities and speaks to the kind of battleground Syria is, a three-dimensional chessboard with multiple players, constantly shifting strategies, and opportunities seized and abandoned on the fly.