A 100kt nuclear warhead airburst will cause moderate damage to an area of 33.5km^2. Beijing has an area of 16800km^2, 501 times larger. China has 160 cities with over 1 million inhabitants. Even if they were all used against major population centers (which they wouldn't be) and all of them made it to their targets (which they wouldn't), thousands of nuclear warheads would still not be enough to destroy a large nation.