In 1979, hundreds of Islamists using weapons smuggled in a coffin seized the Grand Mosque of Mecca. The Saudi military, commanded by the sons of important men, rather than by competent men, proved absolutely hopeless in fighting them. So instead they turned to the French.
The French commandos of GIGN were expert at dealing with terrorist crises, but they were not Muslim and so could not be allowed into Mecca. The solution was simple. The Frenchmen underwent a rapid conversion to Islam and the siege of the Great Mosque commenced. The conversion did not take hold, but the principle remained. An infidel could not enter Mecca, even to save the House of Saud.
John Brennan: From Mecca to Washington | FrontPage Magazine