Let's take a trip back to 1940 to see just how absurd it is to use it as a point of comparison (emphasis added):
The U.S. Army in 1939 ranked 17th in the world in size, consisting of slightly more than 200,000 Regular Army soldiers and slightly less than 200,000 National Guardsmen—all organized in woefully understrength and undertrained formations. The Army possessed only 329 crude light tanks and only a handful of truly modern combat aircraft within a total inventory of just over 1800 planes. It was a force equipped with the leftover weapons, materiel, and doctrine of the last war. It had a grossly overage officer corps, in which advancement was largely a function of seniority. Captains, for example, were usually in their late thirties or early forties. War-related industries were infinitesimal. Congress and the public were united in their staunch opposition to any increased military expenditures or involvements abroad. The mood of the country was distinctly isolationist.
The dramatic changes in the Army's experience, professionalism, hardware, and strength relative to other countries isn't the only reason the comparison is misleading. In 1940, the military broke down as follows:
Marine Corps: 28,335
Air Force: 0 (it hadn't been created yet—that is to say, the army figure includes that era's pilots and air crews)
Now, as history shows, those 458,355 members of the military circa 1940 were sufficient as a base from which to declare war on Japan and Germany in 1941, ramp up personnel, and win that war (alongside allies in Europe, the Soviet Union, and elsewhere). Not that we would've chosen that level had we known the future. Still, if it were the case that U.S. military strength was the same as it was then, it isn't at all clear that we'd have to worry about our national security, especially given the radically superior military hardware available to us today and the dearth of any opponents as formidable as Nazi Germany and Imperial Japan.
That isn't to say that I'd endorse returning to 1940-level personnel strength. And indeed, the military is orders of magnitude bigger and stronger than it was in 1940. That's true even if every proposed personnel cut gets through Congress.
The U.S. Armed Forces Today
As of December 31, 2013, the U.S. Armed Forces was 1,369,532 men and women strong. The breakdown:
Marine Corps: 193,815
Air Force: 329,610
In terms of manpower, if you'd totally eliminated the Army and the Navy on December 31, 2013, the combined total of the Marine Corps and the Air Force alone—523,425 people—would still be significantly bigger than the whole military circa 1940.
Also, if the Army is indeed cut to between 440,000 and 450,000 personnel, as the Obama Administration has proposed, the Army could be characterized as operating with the smallest force "since just before the U.S. entered World War II," but it would also be accurate to say that the Army of 2014 will have 170,977 more people than the Army of 1940. And again, whereas the Army of 1940 encompassed the fighter pilots and bombers of that era, today we've got a whole separate Air Force composed of several hundred thousand uniformed men and women, plus a modern Navy and Marine Corps with significantly more personnel.