Depends on how you define success. While the size of the US military and the geographic isolation of the country certainly diminishes the chances anyone will mount a blitzkrieg against us, there are those who feel that little skirmish in Sept. of 2001 had elements of a successful invasion. The problem might be looking at it through the eyes of the invaded rather than the invaders. We seem to just assume that conquest is the goal when subversion may be more accurate. Attacks in NY, London, Madrid, Mumbai, etc. were generally carried out by foreigners (invaders?) and their actions have lead to at least incremental changes in the behavior of the invaded (success?).