To construct a timeline, the events should be arranged in chronological order. Among the events listed, the earliest one is the formation of the state of Israel, which occurred in 1948.
Here is the chronological order of the events you provided:
- 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)
Therefore, the event that should appear first is the formation of the state of Israel.