A couple of things.
1. As OC already pointed out, our manufacturing isn't really in decline. Other countries are just improving their own manufacturing sectors.
2. As Harry said, the economic interdependence from increased trade makes a large war with another large country, like China, highly unlikely in our shrinking world. Besides going to war with China in the near future is also like the bank you got a home mortgage from trying to burn down your house.
3. The US already has a massive military-industrial complex. We spend more on our military than almost the rest of the world combined. No other country can provide both the quality and quantity of our military's toys. If another country did begin a massive buildup, I fail to see why our complex couldn't keep up again, just like in the Cold War.
4. Even if we do go to WWIII and it doesn't go nuclear within the first few weeks anyway, how is normal manufacturing going to help? Technology has gotten a lot more complex since 1945. It's not like you can go from manufacturing Fords to Abrams as easily as from Buicks to Shermans. I don't see how helpful more non-military manufacturing would even be.