There are a lot of invasions in hisytory which were attempted and then don't happen or fail early on.
Some are even due to simple environmental issues.
Or, y'know, overestimating one's capabilities from arrogance (or underestimating the opponent)... Example: Putin.
Honestly, given what we saw from the supposed elite awesomeness of their supposed vanguard, I'm not sure we would have had much problem if they had attempted something (if being written realistically).