There was a build-up of tensions in the lead-up to the war, not just on the Egyptian side, but also on the Israeli side.

The Egyptians believed that the Israelis were preparing an invasion of Syria, so the Egyptians moved their own force into the Sinai peninsula, in order to deter Israel. However, the Egyptian force was nowhere near sufficient to launch an invasion of Israel.

The closing of the Straits of Tiran was politically significant, but not very economically significant. Only a tiny fraction of Israel's trade went through the straits.

The Israelis had been planning a first strike on Egypt for a long time. Whether they actually feared an imminent Egyptian invasion in June 1967 is questionable. The Israeli military was very confident of its superiority over the Arab forces, and it may have simply taken advantage of the situation to land a devastating blow against the Arabs.

The Israelis were not preparing an invasion of Syria. They were led to believe so by the Soviets; the Soviets were wrong. Egypt and Syria thus mobilized unilaterally.