But my understanding is that it wasn’t just military movement.

Nasser mobilized Egyptian forces in the Sinai; on May 18 he formally requested the removal of the UNEF stationed there; and on May 22 he closed the Gulf of Aqaba to Israeli shipping, thus instituting an effective blockade of the port city of Elat in southern Israel. On May 30, King Hussein of Jordan arrived in Cairo to sign a mutual defense pact with Egypt, placing Jordanian forces under Egyptian command; shortly thereafter, Iraq too joined the alliance.

If I was (objectively) playing a war game - that collection of activities in that short space of time…. that’d be enough of a casus belli for me.