We need to keep whatever can deal with wars, big and small, and economic depressions, big and small. I see China as our biggest threat, both military and economic.
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.