Who could of joined the Central Powers?

Mexico.

Several ways it could come about. Wilson had sent US forces into Mexico against the Huerta regime in 1914, and against Pancho Villa in 1916-17. With either of those in power when the US enters WWI (whenever that may be) they would have little to lose and potential allies to gain by joining the Central Powers.
The Americans were the only major supplier of weapons in the hemisphere.
 
Absolutely true, and it would also have been hard for German arms supplies to get past the Royal Navy, which is why Mexico declined the Zimmermann proposal in 1917. Still, the challenge is only to have a country join the CP -- they don't have to win. Huerta was tempted by a similar German proposal after he was overthrown, and in 1914 he had to fight the US anyway.

http://en.wikipedia.org/wiki/Victoriano_Huerta
 
Top