The airstrike comes at a time when the U.S. presence in Syria has become increasingly visible. Last month, the Trump administration launched the first direct U.S. attack on Assad targets, striking an air base in the central province of Homs in retaliation for a chemical attack that killed scores of civilians in the country's northwest. On the ground, the U.S. presence in Syria is also becoming more visible, as American troops back Kurdish-led forces fighting the Islamic State.