Seems to me that with Willkie, the internationalist, in the Oval Office, many of the same events that transpired in 1941 in OTL would have happened anyway with a different cast of characters. Indeed, Pearl Harbor might have happened sooner (the Japanese had an unknown quantity in Willkie and wouldn't have been averse to testing him?). Since the C-in-C typically stands aloof from military decisions, I would surmise that the battlefronts would have largely gone the way they did, including the promotion of Ike over more senior officers.
The real twist comes with respect to Willkie's health: in OTL, he died in 1944. Possibly with the best care that the Army and Navy have to offer, Willkie might have lived until 1945 or 1946. If I recall correctly, his death was sudder, so we'll assume that at the Yalta conference, Stalin faced a reasonably robust Willkie, and may have been obliged to back off a number of his demands. Thus, I could foresee less of an extension of communism westward than in OTL (possibly even to the point of Germany having only three, not four, zones of occupation).
Assuming Willkie is renominated and re-elected (along with VP Charles McNary), the US, Great Britain, and France

have more of a say in normalizing Europe than they did in OTL, with the Iron Curtain established farther east (certainly including Poland and the Baltic nations; guessing no East Germany). I have to wonder if the GOP would have nominated McNary for a full term: my sense is no; he was from a relatively minor state (Oregon) in terms of electoral votes, so I suspect he'd be a caretaker until 1948, when the Republicans would trot out a blockbuster ticket of Ike and Dewey (for those who would point out that you can't have a presidential and VP candidate from the same state, I'd guess Ike would be nominally have a "home" established in Colorado, where Mamie's family lived). That would blow away anything the Democrats would offer (guessing possibly Byrne and Truman).
With John Foster Dulles running the State Department and Allen Dulles running the CIA, it's not too far-fetched to imagine that the nationalist regime in China would have been somehow propped up until the postwar situation stabilized. No Mao = no "who lost China?" questions = no Joe McCarthy. Fallout from that: California congressman Richard Nixon does NOT attempt a run for the Senate in 1950. Imagine if you will the early 1950s with no McCarthyism...