The shuttle did have a military requirement: the air force wanted to be able to launch into an orbit over the south pole, then up across Russia from the south, to return and land after that single orbit. That pretty much forced the use of wings that could glide, vs capsule style re-entry, and opened the path to most of the shuttle's design flaws.
As for why the air force wanted this, the presumption is surveillance, but I don't think they've ever officially answered, other than denying that they ever considered a bomber mission for the shuttle.
On the other hand Soviets were reportedly scared to death of shuttle starting on a "normal" mission only to use it's wings and heat shield to dip into the atmosphere and change it's inclination, so that the new orbit passes directly over Moscow, from an unexpected angle. Then it would drop a 20 ton worth of RVs with nukes in a decapitation strike without any warning.
It is estimated that this is one of the reasons why the Energie & Buran program were started - they also wanted part of the "fun".
I think the actual shuttle had a life support capability for like 7 people. Not sure of the actual flight crew requirement, there's probably room for like 3-4 other guys. It can land on a runway, but it's gotta be really long, and there's only one shot, and it'll be really obvious and has no ability to evade defenses. No chance of it landing in Moscow unless the Russians let it. And then you'd be in the middle of a huge military base or something, and what are 4 guys going to do except be killed or captured immediately.
Maybe they could get there in an hour. From a specific launch pad in Florida, and if the shuttle is all set up to launch, which takes months. And it'll be blindingly obvious to everyone in the world that something just launched heading straight at Moscow.