They gave orders to commit genocide, and planned its implementation. That is direct enough for our purposes. FDR enabled quite a lot of crimes, but that’s not the same as directing that they be committed. He needed Stalin, so tolerated his crimes, but had he not needed him, he would not have, and he would have been happier to partner with some non-sociopathic thug, if that was an option. Churchill would have been just as happy if there were no refugees to whom he denied asylum in Israel. He was far from an anti-Semite in his personal views. But he, too, was an enabler. Had he done something to save the poor refugees on the Struma, the Wansee Conference might have ended with a decision to call off the final solution. But that doesn’t mean he attended the conference and lobbied for genocide. Stalin, on the other hand, gave orders to liquidate all Jewish partisans, and the non-Jewish partisans were happy to oblige. Stalin himself didn’t lift a hand, but he was the murderer in chief.
I suspect one of the reasons Stalin’s defense of the Ukraine at the beginning of Barbarossa was next to non-existant, is because that was where the majority of the Soviet Union’s Jews lived. Hitler could take care of his “Jewish Problem” in the meantime, before eventually Soviet defense stiffened and ultimately turned the tide of the war.
I didn't know that, do you have links to more information?