So USA cannot join the Axis.
Re-read what Johan wrote: "Home of the Free, which allows them to create factions, but not to join any faction led by another ideology"
Emphasis on *ANOTHER* ideology.
The US starts as a Democracy, so it can (World Tension, Laws etc permitting) join a faction led by a Democracy. If it changes to Fascist, it can join a faction led by a Fascist. If it changes to Communist, it can join a faction led by a Communist.
I read that as just no cross-pollinating going on, so you can't join a Fascist-led or Communist-led Faction whilst still a Democracy.
But, if (say) France has formed its "Little Entente" with the Czechs, or a faction with Italy, and is still a Democracy, the US could (World Tension, Laws etc permitting) join the Little Entente or the Franco-Italian Pact.