First let look at what we might need the Army to do. How about defending Israel from invasion? How about doing regime change on Iran rather than allowing them to go nuclear? How about staving off an invasion of South Korea? Or, in the aftermath of a second Korean War, doing regime change in Pyongyang? How about intervening in some armpit in Africa to prevent another genocide? How about cleaning out pirate bases in Somalia? How about intervention in the Balkans, or some East European armpit?
I'm not saying that we ought to do any of these things, but I do think America needs the capability, just in case. So what does it take to do the job? We did Iraq with 140,000 troops deployed in country. It would take more to deal with North Korea. Let's say we need 200,000 soldiers on active duty, with maybe that many again in the reserves. Modern war is quick, you gotta run what you brung. There is not time to enlist and train troops, the war is over before that happens.
Obama wants to cut the army down to 450,000 men. Sounds like enough? Dunno. The 140,000 soldiers sent to Iraq were all combat troops, infantry, tankers, gunners. Historically, the US Army has a ratio of tooth to tail of about 9 to1. For every combat soldier carrying weapons in the face of the enemy there are nine support troops driving supply trucks, manning depots, cooking, doing paperwork, fixing jeeps, building schools and bridges, etc, etc, ad nauseum. Based on past experience, a 450,000 man US Army might contain only 45,000 real soldiers, which clearly ain't enough.
In actual fact, American troops have plenty of experience in every line of work. Men capable of fighting on the front line, are capable of doing pretty much anything else that might be needed. I suggest that a lot of the specialists behind the lines could be re trained as infantry and sent to the front. Regular units can do much of the work now done by specialists. If we could get the ratio of tooth to tail down to maybe 2 to 1, then maybe 450,000 men might be enough.