Israel launched a pre-emptive attack against Egypt, Jordan, Syria, Saudi Arabia and Iraq which had deployed over five hundred thousand troops, over twenty-five hundred tanks and nine hundred fifty-seven combat aircraft on Israel's borders in preparation for an attack. Israel was outnumbered two to one in infantry, three to one in tanks and three to one in combat aircraft. If Israel had waited for the Arab attack it's troops would have been over run and the Jews would have been exterminated like the Arabs have been promising to do on a regular basis since 1947.
As for not being able to keep territory won in war, Jordan, Syria, Lebanon and Egypt all kept territory they conquered from Israel in the 1948 war. You don't seem to have a problem with that. How about Russia keeping South Ossetia conquered from Georgia, Crimea from Ukraine and the two "breakaway provinces from the current war? You also seem okay with that.