Since we are talking about hardened targets, is there anything that can be used as the nukes are flying over the border? I'm sure various SAM's can work (HQ-2, Patriot, S-300) can take on some nukes. As well, exactly how hardened is a nuclear silo? The United States has been stressing about ensuring the lethality of their Thermonuclear warheads to penetrate those "hardened military silos"?
Let me put it this way. Norad, which built under a mountain (1 mile of bedrock on all sides plus 40-50ton massive steel doors) was expected to survive 30 minutes in a nuclear war. A direct hit from a megaton size warhead will certainly destroy it. You can't "hardened" anything against a nuke. It is far easier to increase a nuke yield than put in more protection.
In the US, the missile silos are designed primarily as the first strike of the nuke triad. They are much more accurate than SSBN's due to the fact they are fixed and know exactly where their launch points are. As a result, their primary targets were military facilities not population centers.
SSBN's are still the best way to ensure survival of your nuke assets than just hardening them.
Manned bombers are the third leg of the triad. They offer the leadership "call back" capability in a nuke war. They are usually scrambled once an inbound nuke is detected. At the height of the cold war, the US had nuke armed bombers airborne 24 hours a day. The bombers today no longer have nuke deterrance as their main mission.
For those who don't know the size of the nukes we are talking about here is rough guideline.
20 kilotons - Hiroshima bomb
0.5 kilotons - bomb tested by North Korea
700 kilotons - average yeild size of US nuke MIRV warhead
25,000 kilotons (25 megatons) - largest ever nuke detonated (USSR)
Edit:
Estimated Russian Nuke arsenal
China
USA
France
UK
As for China, it appears that the strategy is minimal deterrance with zero first strike capability vis-a-vis the US. From the above link:
The only true intercontinental ballistic missile (ICBM) in China's arsenal is the liquid-fueled DF-5, which is capable of targeting the entire continental United States. The exact number of DF-5s is unclear, but Chinese Military Power 2005 states that 20 missiles are deployed in 20 launchers, a number that has remained steady over the past six years or so. The missiles are deployed in silos at two locations, and their nuclear warheads are stored separately nearby.
So these missiles are not on "push-button" launch readiness. But those warheads are big suckers. 3-5 megatons.