U.S. aircraft arrived in Saudi Arabia in 1990, when Iraq invaded Kuwait. They helped repel that invasion and remained at the invitation of the Saudi royal family to help guarantee stability in the Arab oil states of the Persian Gulf despite pledges to Islamic conservatives that they would return home as soon as the Iraq crisis ended.

Still nervous about Iraqi President Saddam Hussein and wary of the future of the Shiite Muslim government in nearby Iran, the Sunni Muslim-run Arab countries of the Gulf Cooperation Council have in the past decade bought tens of billions of dollars in weapons from the United States and accepted what has evolved into a permanent force of American ships, planes, tanks and personnel.

To bin Laden and other extremists who trained with him to fight the Soviet army in Afghanistan, the U.S. presence amounts to a modern crusade, an army of infidels in the sacred birthplace of Islam interested only in oil supplies and defending Israel. It was the basis of his call for holy war against the United States, beginning after the Gulf War.