They're not against invasions... they're against the United States.
It's true that there are pacifists out there who genuinely oppose the use of armed force by anyone. And they tend to be rather rare, especially these days, when anti-war means anti-American.
The opponents of American military action like Ron Paul and Glenn Greenwald invariably find all sorts of excuses when it's Russia that's doing the invading.
The average anti-war activist doesn't wear flowers in his hair. He just believes that America is the source of all evil because it's controlled by...
1. The Military-Industrial Complex
2. The New World Order
... or some other variation on the theme.
When America bombs a country, it's acting on the Vast Conspiracy to seize all the oil, gold or implant microchips in brains for the Neo-Liberal agenda. When Russia invades a country, it's just defending itself against the aggression of the Vast Conspiracy headquartered in the US.
If a country is an enemy of the United States, then it gets a blank check to defend itself by invading and conquering much weaker countries. If it's an ally, then it's also part of the Vast Conspiracy of the Great Satan.
Understanding how these people think is crucial to dismissing their arguments. They're not operating from a moral high ground, they're not against war, they're not against invasions... they're against the United States.
Now if they would just stop pretending that they're operating on any principle more complex than AMERICA ALWAYS BAD, it would simplify things for everyone.