What happened in Russia in 1917 that led the United States to justify its involvement in World War I as a fight for democracy? Russia signed a military defense treaty with France and Great Britain. Russia signed the Treaty of Brest-Litovsk and made peace with the Germans. Russian revolutionaries overthrew the Czar and established a provisional government. Russian Bolsheviks, under Vladimir Lenin's leadership, toppled the provisional government.
"Russian Bolsheviks, under Vladimir Lenin's leadership, toppled the provisional government" is what led the United States to justify its involvement in World War I as a fight for democracy, since the Bolsheviks were communist.