“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.
You must have noticed, no doubt, that Roosevelt failed to oppose the joint Soviet and British action even though he was requested by Reza Shah Pavlevi to do so.
Roosevelt's actions spoke louder than his words (which he also delivered to the Shah).