If the US prioritized the Afghan front over europe or the rest of asia, Petrograd would be even worse. OTL, Truman wanted the soviet people onside, seeing the political battle taking shape (even if his policy for handling it was bad) so he had as many american soldiers as possible evacuate anyone who surrendered and use what technology they could to help with the nuclear health concerns. Personally my favorite quote from him was in this regard came when he was asked how he felt about the fall of the soviets
Is that why they dropped the bomb has high above the ground as they did? Also, I can see why they evacuated so many people in Latvia and Finland when you remember how close Petrograd is. That said, it would later turn out that the bomb over Petrograd was far less severe than people expected. That, combined with preparing equipment to help survivors, meant that Truman was right when Russia became a Republic instead of a monarchy like the UK and Japan made Germany into by restoring the position of the Kaiser.