Following the assassination a month earlier of Tzar Alexander II of Russia, and the subsequent rumors that the Jews were behind the assassination, anti-Jewish riots broke out on the 6th of Iyar. The riots and pogroms lasted for four years, during which time thousands of Jewish homes and synagogues were destroyed, and countless Jews were injured and impoverished. The unrest started out in Southern Russia, and quickly spread throughout the entire country.
Tzar Alexander III actually blamed the riots on the Jews(!) and punished them by enacting new laws which further restricted their freedoms. Among these devastating laws were legislation which restricted Jews from residing in towns with fewer than 10,000 citizens, and limiting their professional employment and education opportunities.
These oppressive laws, known as the "May Laws," compelled many Jews to emigrate. They are said to have caused more than two million Jews to leave Russia, many of them opting to move to the United States of America, and the freedoms it offered.
Link: The Pogroms of 1881-1884