It's the nature of war and conquest. The Israeli government felt is had a reason to invade and hold the land. It did and it is expanding its colonies. That's just war.
I disagree with cjhunsinger's analogy with the US. We did not invade or "take" the 13 colonies from Britain. We were already in the colonies and we kicked Britain out. A better analogy would be the US conquest of the Nueces Strip from Mexico in 1846. Will we or should we ever give that back? I would say we should not have taken it in the first place. But once we did, after 170 years, there is no point in giving it back.