The US didn't give Israel Mandatory Palestine - Britain did.

Well, Britain didn't quite "give" all of Mandatory Palestine, or any of it technically, to the Zionists.

What it did do was first, proactively set a firm date -- at midnight on April 14/15 1948 -- for the Mandate to expire (which it needed to expire soon anyway as the UN was poised to fill the vacuum left by the collapse of the League of Nations). And agree to a pull-out of its forces around the vicinity of the date. Which, while not amounting to a transfer of sovereignty as such to the Zionists, amounted to telling them "have at it", basically.

It had also provided the Zionists with a "moral" mandate that there be some kind of "Jewish home" in Palestine, though that came earlier through various steps (including of course the Balfour declaration). By that time though, the Zionists strill controlled only something like 13 percent of the territory as such.