Unless Congress declared it, it is an illegal war.
Is it an illegal war? Did I miss the declaration?
“Did I miss the declaration?”
You apparently missed all of them.
Starting with North Korean invasion of South Korea, 25 June 1950.
The latest being Russian invasion of Ukraine, 24 February 2022.
From Wikipedia
“No formal declaration of war has been issued in the ongoing Russo-Ukrainian War. When Putin announced the 2022 Russian invasion of Ukraine, he claimed to commence a “special military operation”, side-stepping a formal declaration of war.”