If the British mandate expired with the founding of the UN, what government did the UN put in its place? Truman certainly took a hands-off attitude until the Israeli DoI forced his hand. The USSR was actually much more interested and active in the area until then. Many individual Americans were interested and active, and it really pissed Truman (and the Arabists in the State Department) off. They wanted nothing to do with it and of course the State Department was full of antisemites. The US can’t be accused of any coherent Middle East policy 1945-48 (and maybe not afterwards either).