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Israel did initiate hostilities, which can be interpreted as expansionism. The question, of course, is was the military action justified? I imagine like most everything in this region you can find very smart and educated individuals with very strong feelings and arguments both ways.
More a case of a preemptive attack, posturing by Egypt, the removal of UN peace keepers and the closing of the Straits of Tiran (previously declared by Israel to be an act of war) gave Israel little choice - according to its own beliefs of course. It was probably a war that would have happened at some time given the tensions of the day and the history of the antagonists.
I don't believe the intent was expansionist but more a case of national survival, only the archives may tell us now.