You have to invade someone for it to be an invasion, like Russia is doing in Europe, South Vietnam was not invading anyone, they were merely trying to fight off an invasion.
There is a big difference.
Check out what ole Mr. Webster has to say about it.
in·va·sion
/inˈvāZH(ə)n/
noun
an instance of invading a country or region with an armed force.
That's exactly what we did we sent an invading force to Vietnam. That's it!
Now, of course, you may have written your own dictionary and that's where you get this crazy idea.