What would happen if the United States was invaded?

I have answered this question from many people but I am going to ask this time to see your opinion. It is illegal for the United States Army to be deployed in the US (this is true). So if lets say the Chinese attacked the United States would Russia join in? There was a program that Reagan started that would deflect air missiles (IBCM's,Ballistic Missles) and Russia did not want to do that? Why not? I think Russia was up to something nuclear and was going to nuke the US. There is no reason not to do the program?