Technically, it's not an "invasion"

 (from a comment thread at jabberwocking.com)

"Technically, the troops have been "invited" by the "leaders" of the breakaway regions. Once he recognized the regions, he elevated the "leaders" to International status; they became people who could "speak for" their "citizens". And those "citizens" apparently desire Russian troops to "protect" them from the "Nazi's" living in the Rest of Ukraine.

Allied leaders understand this nuance and certainly expected it. The rubber hits the road when he moves beyond the Line of Control into the parts of Donetsk Oblast and Luhansk Oblast still controlled by Ukraine. If the Allies don't trigger the sanctions at that event they have been bluffing."