FDR had little or no cognitive impairment at first, sure. But by 1945 I think he had a significant degree of impairment, I mean, he had a brain tumor, right? They didn’t feel any need to admit to that. Course it was probably classified. There was a war on. But I think he had obviously declined badly by the time of the Yalta conference.
FDR had cancer and they hid it. Look at the pics of Yalta compared to even a year before, that’s Cancer.