The "Moscow criterion" is one of the underpinnings of British nuclear weapons strategy, devised during the Cold War but still in place to this day, although - perhaps mercifully - no-one was really talking about it until recently, as it has become the subject of review by the government. I imagine, however, it might raise some eyebrows in Moscow.
The criterion is a test for Britain's nuclear deterrent which successive governments have tried to meet, and came to be seen as the key test for deciding whether a British nuclear force was effective or not. Britain's nukes aren't based in missile silos or mounted on massive bombers, like many belonging to the United States, but rather lurk beneath the waves in a class of submarines that we call Trident. One is always at sea, from where the Moscow criterion states that it must be able to single-handedly obliterate the Russian capital.
I've sometimes seen this explained in euphemistic terms such as "must be able to destroy Moscow so as to interrupt Russian control of its armed forces", but that really isn't the point - the point is that it obliterates the Russian capital. This is assumed to provide sufficient deterrent power to stop those pesky Russkies from ever trying to get the jump on us. In nuclear lingo, this is called a "countervalue" attack, where "value" has the meaning of "millions of people", as opposed to a "counterforce" attack, where "force" means the enemy's nukes.
Britain historically wanted to apply the Moscow criterion to its nuclear forces because, even though we depended on the U.S. to some extent for our nuclear forces, and we coordinated with NATO, we also wanted to maintain an independent deterrent. If our nuclear forces met the Moscow criterion, then even if the U.S. sold us out or refused to back us, we could hopefully still avoid getting nuked by the Russians (it may surprise Americans, but the fear that they might someday make a deal with the Soviets and forget our interests sometimes consumed us).
The transition from "counterforce" to "countervalue" wasn't just something that we British underwent, but a global phenomenon after the hydrogen bomb became prevalent in the 1950s. Most people don't realize it, but the hydrogen bomb - first tested by the United States in 1952 and the Soviets in 1955 - was almost as different from the fission devices that were dropped on Hiroshima and Nagasaki as they were from previous weapons. The first fission bomb exploded with the force of 20,000 tons of TNT. The first hydrogen bomb exploded with the force of 10,000,000 tons.
Thus was born the possibility of actually wiping a country off the face of the earth, and nuclear strategy moved from being about an intricate dance designed to knock out the enemy's weapons to a combination of trying to knock out his weapons (with that much firepower, you may as well try anyway) and wipe out his physical existence as a group of people. But we, little country that we are, couldn't have such grandiose goals - we couldn't wipe out the Soviets' nuclear arsenal or their whole territory. So we focused religiously on Moscow, which we rather assumed they were quite attached to.
After the Cold War, the Moscow criterion is certainly open to question, especially at a time when our government is stripping our army away to an empty shell while still spending billions on a nuclear deterrent ("If Lord Palmerston sends the British army to Germany, I shall have the police arrest them," Otto von Bismarck once wryly remarked - Bashar al-Assad might soon make a similar comment). Whether wiping out Moscow or even wiping anyone out ought to be still an option available to British prime ministers, I leave as an open question; but it is evidence of how slowly policy moves on an issue of such importance to all human beings that it has taken until 2012 for the question even to be asked.