US at war? We are past the 60 days for a military operation. From US law perspective, it’s illegal. From the constitutional perspective only congress can declare war which they haven’t.
International law only applies when you ratify a treaty so it becomes domestic law. Thats how treaties work, they actually laws of the country hence the term ratification into domestic law. Why US doesn't violate international law on land mines and cluster bombs. US never signed that treaty.
But US law is not international law. Internationally you are at war, whatever you call it internally doesn't matter to me.