People are killing each other all over the world. Mostly that is what we use our military for. We use it to stop others from turning small killing festivals into big killing festivals.We spend as much a year on defense as the next 15 countries combined.
So 'the largest threat' is really a mute point. If that isn't enough money to keep us safe then how exactly do each of the 15 sleep at night?
They can sleep at night because nobody wants to kill them.
We're the only ones that others want to kill.