WI the Cold War had turned hot? Say that JFK had so handled the Cuban Missile Crisis so...diplomatically, shall we say. There is no nuclear war, but hostilies escalate with the US invading Cuba. Russia responds by taking down America's closest ally, Britain. This is about as far as I've got. I'm kind stuck at the US fighting a 2-front war. If you have any ideas to add, let's hear 'em!
First west Berlin would be invaded, countries like Greece, Iran and turkey are the ones that could easely follow. One of the first things the US feared, should fighting begin, was an invasion of west Berlin. Should WW3 had happened, europe would have been involved for sure, unlike stupid novel where they stay out and us bombers unopposed.
Invasion of england is unlikely, with naval supplies being exposed but an invasion of Norway probably is.
In an invasion of Cuba, there would be significant losses in bombers and other aircrafts to S-75 missiles(offically deployed in the SU as early as May 1st 1957), far easier so than shooting down U-2 planes.
That there where missiles on Cuba able to intercept an U-2 aircraft is what finally convinced the US that a bomber attack on the missile bases, which had 144 S-75s, wouldn´t play out too well.