The second phase of Iranian migration began immediately before and after the Iranian Revolution of 1979 and the overthrow of the Shah Mohammad Reza Pahlavi, and became significant in the early 1980s. As Ronald H. Bayor writes, "The 1979 Revolution and the 1980–88 war with Iraq transformed Iran's class structure, politically, socially, and economically." The revolution drastically changed the pattern and nature of Iranian emigration to the United States, while the Iran-Iraq War that ensued afterwards was also another factor that forced many of the best-educated and most wealthy families into exile in the United States and other countries. Once basically an issue of brain drain during the Pahlavi period, it was now predominantly an involuntary emigration of a relatively large number of middle- and upper-class families, including the movement of a considerable amount of wealth. During and after the revolution, most students did not return to Iran, and those who did were gradually purged from the newly established Islamic Republic. Many students who graduated abroad after the revolution also did not return, due to ruling clergy's repression. As a result, the educated elite who left Iran after the revolution, and the new graduates in the United States who chose not to return home, created a large pool of highly educated and skilled Iranian professionals in the United States. By 2002, an estimated 1.5 to 2.5 million Iranians lived abroad, mainly in North America and Europe, due to the Islamic government's authoritarian practices.