Egypt got mad because of the Suez Crisis a couple years previous (Israel, Britain, and France pushed the Egyptians out of the Suez Canal and face-fucked the Egyptian Army), so Egypt got Syria and Jordan to mass armies against Israel. Israel was like "Fuck no, we're gonna strike before they do", and so they proceeded to shoot down a cargo ship containing most of Egypt's Army commanders, and the bomb the fuck out of everything that moved afterwards.
Jordan and Syria tried to launch counter-offensives, but they got pounded in the ass and Israel ended up taking land from all three defeated nations.