What event led to the Six Day War?

Answer 1

The causus-belli was the Egyptian blockade of the Straits of Tiran. The first military action was the Israeli bombing of the Egyptian Airfields not long after. Depending on whether you take the Israeli or Arab perspective, either of these events could be seen as the "event that led to the Six Day War".

Answer 2

1) Removal of UNEF Forces: As a condition of the Israeli withdrawal from the Sinai Peninsula in 1956, the UN deployed security forces to the Sinai Peninsula to prevent Egypt from massing troops in the Sinai. Nasser evicted the UNEF (the UN security force) from the Sinai Peninsula in contravention of the Armistice for the 1956 Suez Crisis and placed large numbers of Egyptian soldiers and armored divisions on the Israeli-Egyptian border.

2) Arab Military Coordination: Egyptian President Gamal Abdel Nasser coordinated military strategy quite closely with Syria and Jordan. It was clear to many at the time that these forces would not sit idle for long. A number of Syrian battalions were positioning themselves in the Golan Heights above the Sea of Galilee and Jordanian positions in the West Bank were becoming more entrenched.

3) Closure of Waterways: Egypt, counter to the armistice ending the 1956 War, closed the Suez Canal to Israeli shipping and then proceeded to close the Straits of Tiran to Israeli shipping, cutting Israel off from trade with Iran which was Israel's greatest ally in the Middle East at the time and primary supplier of petroleum. This had the effect of strangling Israel and is considered by Israel to be the primary causus belli.

4) Pre-Emptive Airstrikes: Israel realized that its borders were too close to for them to risk an Arab Invasion of Israel before retaliating. Israel would have to fight a pre-emptive war in order to secure the best probability of success in a military engagement. The first act of open warfare was when Israel destroyed 2/3 of the Egyptian Air Force on the tar mac on June 5, 1967.