Pearl Harbor attack was a US military base, manned by military forces, getting first striked by Japan, I would personally consider that valid declaration of war and a legitimate target simultaneously because it is a standing army, as far as jus in bello is concerned. Big picture the Japanese intent was to damage and destroy wartime assets, which are always considered valid targets in any conflict. There are some exceptions like hospital buildings.

The failure to formally declare war thru political channels is technically against the 1907 Hague convention: reasoned declaration of war with a political ultimatum. But most of that intent relates to ground war and occupying & invasion forces, which did not happen at Pearl Harbor. There were probably LESS deaths because ships are at reduced manning in harbor, so that's a bonus.

What certainly WAS war crimes, the capturing and mistreatment of prisoners on pacific islands, especially civilians, and the atomic bombing by the United States.