colonial ownership means nothing in this. the people are the exact same and no one is getting displaced (except very specific situations) whether it be 'owned' by English, French, Portuguese, Spanish, whoever. It was Palestinian land through and through.
Of course the founder of Israel accepted the partition. They were Jewish! I can safely say not ONE Palestinian who owned any of the land before agreed to it. Not one.
The Palestinians were not hated in the region, they had a lot of supporters. So when a people who have done nothing wrong and nothing against the Jews get expedited from their land in a matter of months just so the Jews can have their "Holy Land" which they haven't occupies for ages and ages, of course they're going to declare war against them.
If China invaded the US, you think NATO or any other allied countries would just sit there and let it happen? No.
Although technically this was not an invasion, to the people living there it certainly was. Doesn't mean a thing to the farmer whose house gets destroyed and olive trees burned to the ground whether it's sanctioned by the UN or not. It's all the same in his eyes; the Jews are the oppressors and enemies.
Six-Day war and all that is irrelevant to this. That is just fluff on top. The Israelis should be damned glad they had US support during those campaigns though, or they wouldn't be around in that area any more.