America entering World War I in 1917: Verify a Source for me?

Cougar Hunter

New member
The answer to a test question stated that America entered World War I for three reasons:
1. The sinking of the Lusitania
2. German resumption of unrestricted U-Boat Warfare

And 3. The tsarist government was overthrown which opened the way for American Intervention.

What is up with 3? I've never was able to verify a source for the information. Please check if 3 is credible, or at least reasonable.
 
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