So the basic thrust of your arguement is US forces had self-exploding tanks, bombers who could not hit a beach, and generally incompetent leadership.  I believe you also ranted about WWI helmets, ugly casualty rates, fire bombing of cities, atomic weapons and fearsome enemies flying bamboo airplanes.

So, if the US forces were as incompetent as described why do the WWII nations of  Germany, Italy, Japan and USSR no longer exist?

Seems to me that despite all the faults, failures and just plain incompetence that you have so carefully located, the US soldier, sailor, airman and Marine simply kicked ass.  Something about being willing to say "NUTS".

So my question still remains. 

Why should the US protect Europe, Japan, Taiwan or Korea?  Why should the US protect the Middle East oil fields when the majority of that product goes to Europe, Japan, and China?  What does the US care if heating oil prices in Europe skyrocket?

If you really think the US is going to get involved in your WW3 scenarios, you better explain why the US will be interested.