I know they do but in the case of conflict where does the initial money come from. The president has very broad power when it comes to national security and it seems it would be dangerous for him to have wait on Congress in the case of a threat to the country.
Most of it's already paid for, or appropriated for. The military members are already being paid, the equipment and supplies, weapons, and ammo, are already there for an initial conflict. The only additional cost would be transportation and set up of the bases of operation. Again, the equipment would already be in inventory for an initial operation, and further operations would require further appropriations.