Also, discretionary here is entirely a political/legal term. I think one would find it very hard to argue that military spending is discretionary for any major

Also, discretionary here is entirely a political/legal term. I think one would find it very hard to argue that military spending is discretionary for any major nation in the layman sense of that word, even if the US's military spending is beyond control (on the flip side, the US also gets much less value for its military spending relative to say the value China gets for its military spending).