A good POD could be that: during Great Purge, fearing Stalin's paranoid massacres, Party members and armed forces commanders remove and execute Stalin for "crimes against Revolution", forming a Soviet Junta based on collective leadership's principle after Stalin's strongman posture. Ideally this junta could be based on a five members committee, two for Party (Molotov and Kaganovich), two for Armed Forces (Voroshilov and Tuchavesky) and one for secret police (Yezhov).
The new Soviet leadership refuse to sign the Molotov-Ribbentrop Treaty and so Hitler, after annexing Austria and Czechoslovakia, doesn't dare risk a war with Soviet Union when he is not ready and doesn't invade Poland.
The new Soviet leadership decide to take a tougher approach to Khalin Ghol Undeclared War, tiring to show strongness after the coup: Japan doesn't want to launch itself in militaristic adventures with a Soviet menace over its Chinese territories and find a way to appease temporarily US (as they falsely did in HL, promising peace to Hull shortly before bombing Pearl Harbour).
Roosevelt retires after two terms, then his successor Cordell Hull retires die health issues after one term. His successor (one between McNutt, Kennedy Sr, Douglas, Lucas, Barkley, Byrnes and Truman) loses in 1948 against Harold Stassen. Stassen reinstates Stimson in State Department and relationship with Japan soures.
In Europe Hitler becomes increasingly crazy and dies in 1947, leaving Goering in power as President and Goebbels as Chancellor. The latter allies himself with Himmler and manipulates Goering to search a militar victory to consolidate his regime.
In Great Britain Lord Halifax succeedes to Chamberlein and pushes to continue appeasement with Nazis.
Meanwhile an incident (due their mistaken projects) of Nazi nuclear program reveals their plans about nuclear weapons's creation and starts a nuclear race.
In 1949 the war is about to begin.