Russia's armed forces are but a pale shadow of what they were under the Soviet regime. China has potential, and the advantage of sheer manpower, but will take years to create a truly world-class military. Pariah regimes such as Iraq and North Korea are poor, isolated and imploding. Those nations aren't capable of making war on America. Nor are U.S. allies at risk.
Japan matches China's military spending.
South Korea devotes three times as much as North Korea to its military. And with 29 times the gross domestic product and twice the population of its northern neighbor, it could do far more. Yes, America does remain vulnerable to nuclear attack. That's why research and deployment of a missile defense should be a top priority. Terrorism is an obvious concern, but that's largely the result of misguided intervention abroad.