The United States should demobilize its Army.
Ok, let's agree to keep a couple of small portions, e.g., Special Forces units and sealift capacity (the Army has more hulls than does the U.S. Navy). There may be another group or two of units worth preserving, but what would happen if the United States basically demobilized its entire Army?
Importantly, the odds of any nation invading the United States are almost negligible. No North or South American nation has the capability or evident desire to invade the U.S., regardless of whether the U.S. has a standing Army. Other nations that might want to invade, such as Iran, lack the sea and airlift capability to make an invasion feasible. China, whose intentions toward the United States are more opaque, also lacks the sea and airlift capabilities to make an invasion possible. U.S. surveillance (from space and through other means) would provide timely information if any nation began to acquire the immense sea and airlift capacities an invasion would require. Furthermore, nuclear weapons provide a powerful, almost certainly effective, deterrent against invasion.
The Cold War rationale for maintaining large numbers of troops overseas no longer exists. The United States lacks the resources to fight and to win a conventional ground war against China in Asia. During the Cold War against the Soviet Union, the U.S. troops in Europe were a "trip wire," that if attacked would delay the Soviet conquest of Europe long enough to mobilize more forces and sufficient in number to assure the Soviets that attack would inevitably lead to nuclear war. Thankfully, Russia does not pose a similar threat and there is no identifiable need or benefit from establishing a similar "trip wire" to contain potential Chinese aggression. Japan does not want the U.S. to station troops there. If necessary, the Marines could relocate their forward based troops from Okinawa to Korea, replacing demobilized U.S. Army units.
A small U.S. Marine Corps provides expeditionary capabilities for overseas contingencies. The Air Force contributes to the nuclear deterrent (which the U.S. could also safely eliminate, but that is the subject of another post), vital air defenses, and airlift capability to support overseas operations. The Navy's submarine force provides the critical component of nuclear deterrence (ballistic missile subs are almost undetectable and have sufficient nuclear capacity that the intercontinental ballistic missile and manner aircraft launch platforms are a very expensive, unneeded redundancy). The Navy's carriers and surface forces, in conjunction with the Marine Corps and retained Army units, provide the ability to project power.
Demobilizing most of the Army's authorized end strength for fiscal year 2012 of 547,000 (keeping only 10%, say) would represent a huge savings of perhaps $100 billion with no reduction in the nation's safety. Reducing the size of the Army reserve and National Guard (together totaling almost 600,000 personnel) would generate smaller but still very significant savings.
This proposal is not anti-defense but pro-United States. The U.S. has huge unfunded and underfunded requirements that include repairing a crumbling infrastructure, soaring national debt, spiraling healthcare costs, and an educating system that is increasingly uncompetitive. Spending limited resources in ways that develop the nation will produce more benefits than wasting money on preparing to fight a massive ground war that the nation will never need to fight. Realistic mission appraisal and appropriate reliance on unmanned weapons systems will also permit substantial reductions in the size and force structure of the U.S. Navy, Marine Corps, and Air Force.