So, any arguments to justify it are invalid.
So, any arguments to justify it are invalid.
This is correct.
Regardless of the purely internal characteristics of a sovereign nation, as long as they keep their naughties "in house", any other nation attacking it militarily, especially with the goal of territorial expansion, automatically becomes the "bad guy", and any defensive action, including counter-invasion, is justified.
Your argument is literally missing a premise, and that can be taken in two ways.
Just because an argument is invalid does not mean that its premises are all therefore necessary invalid. Some or all of them may be invalid, or an entirely valid set of premises may be put together in an invalid arrangement.