Khur
Sympathy for the Devil
Geopolitical Maneuvering
One point I'd like to add is that if nobody took credit, the powers that be in our country would then start searching and pointing fingers where they liked. Thus, everyone who'e irritating to our foreign policies and commercial interests comes on the screen as a potential threat. Then, the government uses this as an excuse to attack or engage in diplomacy coupled with other powerful countries to pacify troublesome countries.
There are some who speculate that the 911 attacks were exactly this sort of thing. US attacks self (using American infrastructure and Islamic extremists), and the reason the attacks get through (despite all the protocols to protect from such a thing) is because the US wants them to. Result: a powerful world political posititon to pacify "threat" states, a sympathetic world political climate, a public galvanized by fear to accept government policies that could never be enacted without such fear. Worth 3000+ lives? Ask Sun Tzu.
Now take into account your scenario multiplies the death toll by more than 300!
These points are especially true if a government agency detonated the weapon, or if some cover-up was needed and implemented. As our own time shows, there must be someone to blame. Observe, and you find this is true all the way from local law enforecement and state courts, to the top of the political food chain.
Thus, while the threat of nuclear war would be averted, the US (and united allies, among whom might be found Russia and China after such a horrible atrocity) would indeed go to war. Soft states in Asia like N. Korea of today, would eventually be absorbed diplomatically by the veil of an increasingly capitalist China, pacified by economic bounty--similar things could happen in Eastern Europe, though Russia doesn't have the power, wealth, or influence of China anymore.
Harder states would see war, reconstruction, puppet governments, and finally corporate investiture.
Just some thoughts.
One point I'd like to add is that if nobody took credit, the powers that be in our country would then start searching and pointing fingers where they liked. Thus, everyone who'e irritating to our foreign policies and commercial interests comes on the screen as a potential threat. Then, the government uses this as an excuse to attack or engage in diplomacy coupled with other powerful countries to pacify troublesome countries.
There are some who speculate that the 911 attacks were exactly this sort of thing. US attacks self (using American infrastructure and Islamic extremists), and the reason the attacks get through (despite all the protocols to protect from such a thing) is because the US wants them to. Result: a powerful world political posititon to pacify "threat" states, a sympathetic world political climate, a public galvanized by fear to accept government policies that could never be enacted without such fear. Worth 3000+ lives? Ask Sun Tzu.

Now take into account your scenario multiplies the death toll by more than 300!
These points are especially true if a government agency detonated the weapon, or if some cover-up was needed and implemented. As our own time shows, there must be someone to blame. Observe, and you find this is true all the way from local law enforecement and state courts, to the top of the political food chain.
Thus, while the threat of nuclear war would be averted, the US (and united allies, among whom might be found Russia and China after such a horrible atrocity) would indeed go to war. Soft states in Asia like N. Korea of today, would eventually be absorbed diplomatically by the veil of an increasingly capitalist China, pacified by economic bounty--similar things could happen in Eastern Europe, though Russia doesn't have the power, wealth, or influence of China anymore.
Harder states would see war, reconstruction, puppet governments, and finally corporate investiture.
Just some thoughts.