It's around 50% of discretionary spending. It's a smaller but still sizable portion of the overall budget once mandatory spending (i.e. Medicare, Medicaid, and Social security) is included.
I wouldn't put it past this fucker to start a war just before 2020 elections to try and leverage that as a reason for him to be re-elected. Isn't it no President running for re-elections during a war has ever lost or something like that?