What was President Wilson's biggest mistake in intervening in the Mexican War in 1914?

It is not
possible to say objectively that President Wilson made any mistakes in his policy towards Mexico
in 1914.  You can argue that he made mistakes, but you cannot objectively prove that he did. 
Any statement about which of his actions was his biggest mistake is simply a matter of
opinion.

One possible view is that Wilsons biggest mistake was probably his
actions in the Ypiranga Affair.  In 1914, Victoriano Huerta was ruler of Mexico, having seized
power in a coup.  The US disapproved of the way Huerta had come to power and it disapproved of
his dictatorial rule.  Therefore, Wilson supported Huertas...