Not least in importance was the money Saudi Arabia earned from exporting oil. The pumping and export of oil from Saudi Arabia started during World War II, and its earnings helped fund religious activities in the 1950s and 60s. But it was the 1973 oil crisis and quadrupling in the price of oil that both increased the kingdom's wealth astronomically and enhanced its prestige by demonstrating its international power as a leader of OPEC. By 1980, Saudi Arabia was earning every three days the income from oil it had taken a year to earn before the embargo. Tens of billions of US dollars of this money were spent on books, media, schools, scholarships for students (from primary to post-graduate), fellowships and subsidies to reward journalists, academics and Islamic scholars, the building of hundreds of Islamic centers and universities, and over one thousand schools and one thousand mosques. During this time, Wahhabism attained what Gilles Kepel called a "preeminent position of strength in the global expression of Islam". The "apex of cooperation" between Wahhabis and Muslim revivalist groups was the Afghan jihad. In December 1979, the Soviet Union invaded Afghanistan. Shortly thereafter, Abdullah Yusuf Azzam, a Muslim Brother cleric with ties to Saudi religious institutions, issued a fatwa declaring defensive jihad in Afghanistan against the atheist Soviet Union, "fard ayn", a personal (or individual) obligation for all Muslims. The edict was supported by Saudi Arabia's Grand Mufti (highest religious scholar), Abd al-Aziz ibn Baz, among others. Between 1982 and 1992 an estimated 35,000 individual Muslim volunteers went to Afghanistan to fight the Soviets and their Afghan regime. Thousands more attended frontier schools teeming with former and future fighters. Somewhere between 12,000 and 25,000 of these volunteers came from Saudi Arabia. Saudi Arabia and the other conservative Gulf monarchies also provided considerable financial support to the jihad – $600 million a year by 1982. By 1989, Soviet troops had withdrawn and within a few years the pro-Soviet regime in Kabul had collapsed. This Saudi/Wahhabi religious triumph further stood out in the Muslim world because many Muslim-majority states (and the PLO) were allied with the Soviet Union and did not support the Afghan jihad. But many jihad volunteers (most famously Osama bin Laden) returning home to Saudi Arabia and elsewhere were often radicalized by Islamic militants who were "much more extreme than their Saudi sponsors". The February 1979 Islamic Revolution in Iran challenged Saudi Wahhabism in a number of ways on a number of fronts.