To determine which event should appear first on a timeline, we need to look at the chronological order of the events listed:
- The formation of the state of Israel (1948)
- The Iran hostage crisis (1979-1981)
- The Persian Gulf War (1990-1991)
- The 9/11 attack on the World Trade Center (2001)
Based on this timeline, the event that should appear first is the formation of the state of Israel.