The Islamic Republic of Iran began with the Iranian Revolution. The first major demonstrations to overthrow Shah Mohammad Reza Pahlavi began in January 1978. The new theocratic Constitution — whereby Khomeini became Supreme Leader of the country — was approved in December 1979. In between, the Shah fled Iran in January 1979 after strikes and demonstrations paralyzed the country, and on February 1, 1979, Ayatollah Khomeini returned to Tehran to a greeting by several million Iranians. The final collapse of the Pahlavi dynasty occurred shortly after on February 11 when Iran's military declared itself "neutral" after guerrillas and rebel troops overwhelmed troops loyal to the Shah in armed street fighting. Iran officially became the Islamic Republic on April 1, 1979, when Iranians overwhelmingly approved a national referendum to make it so.