1946: Canada Develops Nuclear Weaponry.
Most butterflies aren't really flapping yet. However, with maybe more nuclear scientists divided the major butterflies are some differences in security around the people who got the Soviets the tech needed. So, by the Korean War the only nuclear nations are Canada and the USA. Without fearing USSR's retaliation just yet, these weapons are used in the conflict. Korea is forced back into the Status Quo. However this leads to a much more... liberal use of nuclear weapons. Before long ICBMs are being thrown because of the NATO intervention in the Chinese Civil War Redux and although the USSR doesn't have enough nukes they (as well as China) have enough chemical and biological weaponry to finish the job.