US government debt doesn't exist as a line-of-credit agreement that someone could choose whether or not to "call in". It's primarily represented by Treasury bonds, securities which represent a promise by the US government to pay a specific amount of money at a specific point in time. It's true that the US can decide one day to default on these promises, but this doesn't have anything to do with military strength, nor can military strength mitigate the negative consequences for the (mostly domestic) investors.