How about the Mexican-American war, where US soldiers crossed the Mexican border, Mexican troops chased them away, the US decided this was a Mexican invasion, the US declared war, then we took the entire South-Eastern United States from Mexico. All because WE invaded THEM first.

Ah, but it was the Manifest Destiny of the USA to conquer North America and Mexico happened to be in the way. How much more justification do you need?

(Note, I grew up in Canada, which became a single country for mutual defense in case the USA wanted to repeat the incident on its northern border. Details are a bit more complicated than that, but that is the gist of it.)

How about the most recent US-Iraq war? The one justified by "OMG WMDs and Al-Qaeda", and a vial of scary stuff waved around at the UN by our Secretary of State?

Merely saying a few things and seeing what might stick in order to justify your actions doesn't automatically qualify them as justified or reasoned.

One can justify anything. However narrowing that statement to "for difficult to justify reasons" leads to list of wars likely in double figures.