Timeline of the Syrian civil war and the United States’ response

The U.S. attack on a Syrian air base came after years of heated debate and deliberation in Washington over intervention in the bloody civil war.

Chemical weapons have killed hundreds of people since the start of the conflict, with the UN blaming three attacks on the Syrian government and a fourth on the Islamic State group. One of the worst yet came Tuesday in rebel-held northern Idlib and killed dozens, including women and children.

That attack prompted President Donald Trump, on day 77 of his presidency, to dramatically shift U.S. policy, with the first direct U.S. attack on the Syrian government.

WATCH: U.S. launches dozens of missiles targeting Syrian airbase

1:06 U.S. launches dozens of missiles targeting Syrian airbase