How would you explain the six-day war?

If I'm not mistaken... Israel went against Syria, Jordan, Egypt who had a bunch of other Arab countries backing them. So obviously were far outnumbered, yet Israel won and was able to take control of Gaza, West Bank, Sinai from Egypt, East Jerusalem from Jordan and the Golan Heights from Syria. How did it happen? Was Israel that technically advanced even back then? Also, how on any basis can the Arab countries demand the land they lost (badly) in a war? Since when did losers get to "have".