Remember, the Ottoman Empire fell and all this oil rich real estate was there for the taking.
“Carve up what? The Middle East had been carved up at the conclusion of WWI, 20 years earlier!”
At the end of WWI the infant USSR was not a great power. In WWII it was, and England could not have easily repulsed a Soviet push into Iraq. They had to negotiate with the Soviets and they did.
Not to mention that FDR showed far more deference to ‘Uncle Joe’ than he did to Churchill, which England also had to consider.