No legal (domestic or international) argument exists for the Syrian strikes.

The authors are Jack Goldsmith and Oona Hathaway, both lawyers, and they throughly disprove all legal “arguments” for the Syrian strikes presented in the past several days.
Not that concerns for enforcing International law motivated Trump (nor should it) but it is hard to resist the temptation to point out the irony of a state to violate international law in pursuit of enforcing it.
You can read their blog post here.