Many people think that the United States needs an enemy to rally people and the Congress behind large defense budgets and an assertive foreign policy. At least since World War II, there has always been a ready candidate -- first the Soviet Union and International Communism, then Japan [in economic terms only], then China, then Iraq and Terrorism, and now, according to Gallup, Iran. Despite these public attitudes, the policymakers still focus on the only nations that truly can threaten America with sizable nuclear capabilities, China and Russia. That's what the DNI has testified.
Even with China and Russia, the most important question is intention, not just the capability. That's always harder to know, but I sure don't lose sleep over Russian, Chinese, or Iranian nuclear attacks on the United States.