The following quote is a partial answer to your first question. The basic answer to your question can be found in the founding documents of the Jewish State (I refer you to the San Remo Resolution, the British Mandate, and the Franco-British Boundary Convention of December 23, 1920.)
According to Ambassador Dore Gold, "International jurists generally draw a distinction between situations of 'aggressive conquest' and territorial disputes that arise after a war of self-defense. Former State Department Legal Advisor Stephen Schwebel, who later headed the International Court of Justice in the Hague, wrote in 1970 regarding Israel's case: 'Where the prior holder of territory had seized that territory unlawfully, the state which subsequently takes that territory in the lawful exercise of self-defense has, against that prior holder, better title.' Israel only entered the West Bank after repeated Jordanian artillery fire and ground movements across the previous armistice lines; additionally, Iraqi forces crossed Jordanian territory and were poised to enter the West Bank. Under such circumstances, even the UN rejected Soviet efforts to have Israel branded as the aggressor in the Six-Day War."
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