Rather than some blowup like al Qaeda smuggling in nukes, a more likely scenario would be something much more mundane and stupid, e.g. a political gridlock between the President and the Congress, as happened briefly back in the 90s between Clinton and the Newt Gingrich-led House. Various government activities were shut down. It never got far enough to lead to not paying interest on the national debt, but it would not take all that much more of a conflict to do it, especially if we had pressure from abroad, with our humongous foreign debt. It has long been a ridiculous joke that Moody's downgraded the quality of Japanese bonds, when they sit on nearly $1 trillion in forex reserves, while US securities get top rating, while the US owes over $3 trillion abroad, a substantial chunk of that to the Japanese.