Operation Desert Shield/Desert Storm(1990-91): America's first encounter with Iraq's Saddam Hussein involved 2.3 million military personnel, of whom 2.2 million are still present.

Melanie Stetson Freeman / The Christian Science Monitor / File
In this 1998 file photo, Soviet-made Iraqi tanks from the '91 Gulf War rust in a military graveyard on the outskirts of Kuwait City, Kuwait. Many tanks bear US soldiers' grafitti such as this one: 'Hi Mom.'