That's not neccessarilly true, Tex.
As has been pointed out, each nation has (as far as I know) a pre-defined government type - democracy, facist or communist. No matter how much you influence them, once your threat to them becomes too high they will join the faction that suits their philosophy.
For example, in my previous (aborted) game, I had influenced Poland almost 80% of the way to the Axis "corner," but on my annexing Czecheslovakia, they immediately joined the allies because my threat level to them had naturally gone through the roof.
I have seen (but not had time to read) a couple of threads that mentioned the US joining the Axis, and to be fair, this should be a possibility.... If a very, very remote one. In just the same way, if playing as Germany, ignoring the US should produce a very slight possibility that the US will perceive the Axis as a threat and join the Allies early. The system isn't broken as such, but may need a slight tweak to make bigger nations such as the US who start the game unaligned drift slower and react less to diplomatic efforts on the part of the player or the AI.
Pre-war the US was determined to remain neutral, and if the influences from Europe had been different (such as the if the Germans had indeed used propoganda to successfully raise the perceived threat to world order the Britain and France represented), and had Hitler not been so monumentally stupid in declaring war on the US after the Japanese attack on Pearl Harbour, there is a strong possibility that the US wouldn't have entered the European war as early as they did.
Personally getting the US to join the Axis in my game isn't one of my aims..... What I'm trying to do is "suck up" to the US to keep them out of the Allies for as long as possible!