What do you call an invasion?

The most popular term is pre-emptive war. Since you're preparing to attack us, we have to invade you to prevent a war. Nazis used this term to launch their Operation Barbarossa against Russia.

A variation of pre-emptive war: Self-defense counter-strike. This is a term invented by Chinese when they invaded India and Vietnam decades ago.

Liberation, another abused term used quite often. The US forces invaded Iraq five years ago under this term. So did many other powers. India and Argentina used when they invaded the Gao and Falklands island respectively.

Peace enforcement operation. A laughable term invented by Russia when they invaded Georgia a week ago. Incidentally, the Russians claimed that they only wanted to "cool down Georgia's hot-heads" when their jets flew over the region.