 Complete the following phrase:  These massive uprisings, which shook Soviet power in Eastern Ukraine, Tambov, and Siberia in 1919-1921, involved more than 100,000 guerrillas and millions of peasant supporters. In the words of Vladimir Lenin, it was a great internal political crisis, the first in Soviet history. These rebellions were recognized by Lenin as far more dangerous than all the forces of Denikin, Yudenich and Kolchak combined, as he proclaimed on March 8, 1921, in his opening speech at the 