Well, it kind of depends on how you’re measuring. Are they attacking the US on the homeland without the aid of Canada or Mexico? In that case the terrain around the US is going to be a death trap. Any troops will land on hostile shores and quickly be mired in various mountain ranges.

But ultimately I’m not sure if it’s really that interesting of a question, outside of a “what if?” scenario. Armed forces exist in the same world as diplomacy, and the US is on good terms with many of the top ten. The big hope is that there can be military alliances that are one sided enough in size that no one wants to test the water.