That's an easy question. If the war/invasion is started and...

That's an easy question. If the war/invasion is started and being conducted by the USA/NATO/EU/UK/Ukraine and their allies then the war/invasion is legal. If the war/invasion is being conducted by Russia/China or any of its allies, then it's illegal and if the war/invasion is conducted by a country NOT falling into any of those categories then whether it's legal or not depends on what America decides at the time and from time to time. See, its simple really. Its not rocket science.