TL;DR: President Huey Long turns America's post-Depression economy into Scandinavia, and a different WWII establishes a different NATO that an unbothered Canada doesn't join, causing it to develop nationalist and isolationist tendencies.
Somehow, an assassin kills FDR during his inaugural address, leaving Vice President Huey Long in charge. The "People's Deal" goes through and is incredibly popular among the destitute masses, although the heavily redistribution-ist program pissess off more than a few members of the business community. Under Long's very progressive welfare program, a basic income is instated for all the poor and middle-class Americans at the expense of the rich. After a few years (circa early 1940), Corporate America's had their fill of Long's agenda, and begins conspiring with the NSDAP of Germany, and a Nazi infiltrator (later thought to be an acquaintance of Henry Ford's) drives a car bomb into Long's motorcade. Long survives, but the revelation of the attacker as a German prompts the US to declare war against the axis. However, America's executives are much less enthused about conversion than they were under FDR in OTL, which is saying something, and with an extra year of prep time knocked away, the European front is much bloodier for the US. However, thanks to the Corporate-NSDAP collusion and the subsequent attack on President Long's life, Japan is scared away from launching the Pearl Harbor attacks, and as a result, they continue their campaign through the East, eventually taking over China.
The Soviet Union comes to the rescue as in our OTL, and the European Theater is narrowly won for the Allies. In the Pacific Front, Japan has near-total control of Asia, and has begun moving into southern Soviet Territory in 1946. After three more years of fighting in the Chinese Theater between the Allies (now with less US commitment and the help of Chinese rebels) and Imperial Japan, the joint Soviet-American Nuclear Program comes to fruition over Hiroshima and Nagasaki. The Shogunate agrees to cease any hostilities, but it won't relinquish any territory. The Allies, tired of the decade of bloodshed, reluctantly agree.
There's no Cold War that we'd recognize, instead, the world is tripolar, with The United States, Soviet Union, and Japanese Empire as the three major powers. Nothing ever goes nuclear, but there's some military scuffles between the USSR and the Shogunate over the border of the Country Formerly Known as China. The U.S., finally enjoying economic prosperity (the economy looks like what we'd consider the Nordic Model) lets the other two wear each other out like a couple of Siamese Fighting Fish. We use our status as the only country whose manufacturing isn't destroyed and/or still in war mode to create a sphere of influence in Europe, the North Atlantic Prosperity Organization, which is still kicking in the 2010's.
Canada, largely unaffected by WWII, sits the negotiations out and does not join NAPO. The NAPO agreement members are miffed at Canada, and take their ball and go home. Canada develops a paleoconservative and economic nationalist streak, which persists into the modern era.
As of 1952, Huey Long looks at the idyllic economy and the improved world order and decides not to run again. The first thing his successor, President Dewey does is sign the 22nd Amendment, preventing anyone else from sitting behind the Resolute Desk for two straight decades.