Comment by Georgelemental

> That wasn’t part of the partition plan, and hadn’t happened yet.

I must admit I don't know as much as I would like about this, so please correct me if I say something stupid (and share reading recommendations), but: my understanding is, "Israel" at this point was composed of several factions who disagreed greatly about methods. Probably Ben-Gurion did not have ethnic cleansing in mind, but Irgun and Lehi did, which was enough—the most radical factions were also most willing to fight, which gave them outsize power. (Though even Ben-Gurion did not intend to stay only within the borders allotted by the UN, he intended to take control of even Jewish settlements outside the partition borders.)

As for the war: the invasion by the Arab countries upon Israel's declaration of independence was an expansion of a civil war within Palestine that had already been going on for more than 5 months, and that the Palestinians were decisively losing. The Arab states hoped mostly to save face and to stop the flood of Palestinian refugees that would result if the current trajectory continued.