Well it won't be Japanese ever again and I suspect all countries feel this way -
"You cannot invade the mainland United States. There would be a rifle behind each blade of grass." - Yamamoto
Though if Russia, China and some other allies all decides to blitz we could be fucked. Though I can tell you they will get one hell of a fight
Any invasion would coincide civil war.
In fact it's amazing how easy another civil war could really fuck up america. Between the mexicans, blacks, racists, bible thumpers, anti government types, libtards, organised crime, and the rich-poor divide it wouldn't take much for things to get explosive. Add in massive geography and a people desperate for stability and anything could happen.