There have been a series of Israeli-Arab wars most notably in 1948–49, 1956, 1967, 1973, and 1982. It is hard for me to say which was the most crucial. I think all wars are horrible. I am going to say I think the 1973 war was one of the most crucial because it resulted in a cease-fire agreement between Israel and Syria that also covered separation of their forces by a UN buffer zone and a prisoner of war exchange.
I also think the peace treaty signed by Israel and Egypt in 1979, formally ending the war that began in 1948-1949, was significant.