I prefer the second option on socio-political grounds:
"He feared U.S. invasion"
Because, as opposed to an attack, an invasion is a drawn out process, and the indefinite article excessively encapsulates it. I would compare this to saying,
I fear an afterschool detention is in store for me
I fear punishment is in store for me
While my punishment could be a detention, the idea of punishment doesn't imply a finite action like a one-hour detention.
You don't lose any meaning between "a U.S. invasion" and "U.S. invasion" but, contextually, I imagine that a person who fears invasion does not only fear the finite action that occurs when borders are breached but the attendant occupation of the territory, which comprises a drawn-out and nearly undefinable period of time--perhaps, even a permanent condition owing to the leftover cultural and religious residue from the expelled invaders.