Have a POD where both the USSR and US deploy nuclear weapon armed satellites in orbit
A camouflaged fobs system on spy-satellites:
Might be the perfect way for a first strike on c4 (more on the United States, because of their more high tech c4) done by EMP and some deep penetrating warheads. This could be done by camouflage reconaissance satellites suddenly releasing deep penetrating warheads in rentry vehicles on communications centres and by detonating high yield warheads in the upper athmosphere to create an emp effect. There would be nearly no early warning and suspiscion, beacuse these reconaissance satellites are flying quite low and would overfly naturally these c4 centers on a regular basis until they reveal their true nature.
Shortly after that, the bomber bases, missile fields and ssbn bases would be taken out by ss-18 and slbm.
Decapitation strike would be also feasible, but maybe not prefered for political reasons, because after the counterforce first strike there has the be someone left for blackmail on countervalue afterward.
The answer, if you're just going to ignore the Outer Space Treaty is geosync orbital platforms. Coupled with high-acceleration missiles (like a space-based version of the Sprint) with city-buster warheads and radar-absorbent coating, and parked over the longitude of you enemy, you could actually get the time-to-target close to an ICBM (or even less, depending on the quality of the heat shield), but with potentially less warning. That would be something close to the orbital weapons in 2001.
The reason the Soviets never fully exploited the loophole regarding FOBS is because of the instability orbital weapons brought. Simply put, sattelite-based weapons shortened response times so much that it would result in a situation rather comparable to a hostage situation stand-off with a extremely stressed hostage taker... even the slightest movement could result in a bloodbath.
The Soviets figured that the time-advantages gained in orbital weapons wasn't worth pushing their foes into a state-of-mind which would make accidental nuclear war a very plausible scenario.
How much does the orbital weapons shorten the response time?
The problem with nuclear armed satellites is that everyone knows where they are, and thus can station several conventionally armed satellites near them. Also, in LEO you have to have a barrage of the things to ensure one is always over the enemy (at least, when the enemy is smallish, like Britain or France), or if they're in Geosynch Orbit you need to attach massive boosters to ensure arrival times of minutes rather than hours.