Trump's missile strike on Syria required Congressional approval, believes Andrew Rudalevige of the Washington Post. He documents that during the past 30 years each US president had to cite humanitarian matters or treaty commitments to justify military engagement in other countries. Congress blocked Obama from intervening in Syria in 2013 after a chemical attack similar to the recent one. Rudalevige infers that Trump did not have a better reason for his assault. According to him, this decision wasn’t made in self-defence or to preserve peace or stability, so it was illegal.
Trump’s Syria strike was legal because it was in US national interest and not an outright declaration of war, writes Ryan Lizza of the New Yorker. He cites Harold Koh, a former legal advisor to the Obama administration, who argues that since it was a single strike that won’t continue, it cannot be labeled as an act of war. It therefore didn't require Congressional approval. The strike came in response to the Syrian government’s use of chemical weapons, and aimed to prevent the destabilisation of the region, both of which are in the US’s national interest, claim Lizza’s sources.