If there is a WWIII, then the US will not be involved.
Most of our heavy industry (and it's polluting byproducts) are overseas now, there are only two large shipyards, for example in the US, both subsidized by the government and mostly building military ships and submarines. The steel industry in the US is a shadow of what it used to be. Manufacturing is overseas, with even the aircraft industry, arguably one of the strongest US industries, even sub-contracting major assemblies outside of our own borders (for example, the new Boeing 787's wings are made in Asia).
The US gave up making the tools that make other tools...we can design them, we can re-build them, but it would take a few years, and a "world war" won't last that long.
The EU lacks the political will to get into a war.
The PRC is probably the only rising power with a self-sufficient economy, however it's population is beginning to get used to prosperity, something the government can't provide without export $$'s.
India and Pakistan, All of Africa, maybe parts of the former Soviet Union (but not Russia) might fight each other. North Korea has nothign to loose, but isn't powerful enough to suck the entire world in. A fight yes, but a "World" War? Nawwwww.
-SB