The Constitution provides that Congress has the power to raise armies, and declare war. The President is the commander in chief. Nothing needs to be changed to allow us to defend ourselves and maintain a world presence.
As to dealing with economic depression, federal involvement in the economy sometimes creates them, and involvement in mitigating them only makes them worse. A good primer for you would be "The Forgotten Man". Get it on Amazon, read it, and then see if you still think we need the federal government to "deal with economic depressions, big and small". If you do, then you have been in the wrong place for 13 years. You would be a follower of JM Keynes.
While you are at it, brush up on Hayek, "The Road to Serfdom". I say this not to be condescending, but if you have not read it, you may not understand the basis of my positions. If you read it and you are still Keynesian, then we have issues. You and Krugman would be closer politically.
Hey I agree with Keynes to the extent 'that in the long run we will all be dead'. Market solutions that say 'eventually' don't work in a political democracy where the long run ends on election day and people vote on how well they are doing - majority wins and the worse it is, the more non-conservatives win. As far as war goes I want a government military that is too big too fail. My eye is on China and we won't have time to mobilize, speed is of the essence.