The Arabs attacked in 1948, not 1945, after Plan Dalet, when Ben Gurion began the massacres and expulsions of natives.
Yes, just as Iraq's invasion was repelled, so should Israel's.
The legal borders are those of '49, when Israel was accepted in the UN after making promises it still has not kept.
Do you really not know that Zionists always planned to take the whole of Palestine?
“After the formation of a large army in the wake of the establishment of the state, we will abolish partition and expand to the whole of Palestine." — Ben Gurion, p.22 “The Birth of Israel, 1987” Simha Flapan