Attacking a military ship is generally not a war crime (as defined by international law such as the Geneva treaties, Rome Statute etc...). It is an act of war (same as invasion or bombardment of another country), and is likely to see retaliation by the attacked country.

Aggression (i.e. unprovoked acts of war) is against the Charter of the United Nations, which also includes the International Court of Justice as a dispute resolution mechanism. It is up to the United Nations Security Council (at which the US has a veto) to authorise enforcement of ICJ rulings.

If a nation is acting to protect another nation facing aggression from the US, it would be legal for the attack US military ships. The reason why they wouldn't would more be that it would likely bring counter-retaliation from the US.

If you want to start a war with the US, sure.

Rules for thee not for me

Legal by whose standards? The international law’s? No one enforces that. Unless it’s to benefit the richest. Most international law is followed basically on the honor system.

By the US’s standards, everything is legal because the president has no limitations because our government will never pass those limitations on a president. If it was illegal, no one is can or will enforce it.

Who cares about “legal”?

A war is a war whether the aggressor decides to officially call it that or not

War - Noun -fighting, using soldiers and weapons, between two or more countries, or two or more groups inside a country - Cambridge Dictionary

It's just as much a war as Putin's war in Ukraine is. The fact that Congress hasn't gotten round to declaring it yet is moot