The nation has been the center of various empires over the past two thousand years, including the Median, Persian, and Islamic Empires. The region came under increasing Western influence in the late 19th and early 20th centuries, most notably Britain, who controlled much of the nation's vast oil reserves until after World War II. In 1979, the Persian Shah, Mohammad Reza Pahlavi, was overthrown by Ayatollah Khomeini, who established the nation's current Islamic Republic and held a group of Americans hostage at their Tehran embassy for over a year.