Discretionary as in military pensions, interest on the money borrowed to finance the military, money for wars rather than regular military spending, money for defense projects that do not appear in the budget because they are classified, ... . Defense projects through NASA, the Dept of Homeland Security, etc..

One can easily reach more than half the $4T total budget with expenses solely tied to past and present military spending.

The Constitution mandates there be no limit on the budget for the military? That is news to me.

And fantasies that the US will perpetually keep Europe, Asia, and the Middle East cowering in terror with our conventional and nuclear arsenal are of little comfort to me. Any truly aggrieved country can decimate the rest of the world any number of ways now, with chemical, biological, or nuclear means, the days of brute-force bullying are fading. We can push around unpopular tinhorn dictators in small countries, but that only goes so far. The rest of the world can laugh as we bankrupt ourselves (and China certainly is now).

It always amuses me that we so value one-man/one-vote at home, but all those people outside our borders are simply supposed to obey our "policemen", and not have a voice in how the world as a whole works. Everyone is equal, so long as they are a US citizen.