Why does the us maintain such a large army?

To allow for the protection of the free world. Not only does it allow us the means to hinder worldly aggression but it also prevents potential aggressors from perceiving of the United States as an easy target.

    I hope you do not take offense,
    To that that's gone before;
    'Tis only that it's my two-cents,
    And not one penny more. §;o)

One is for our defense, but it is way overkill. The main reason is the money defense contractors make. Eisenhower (the last great republican president) warned of the power and influence of the “military industrial complex) in the 50s. He saw the power they had after WWII, and then during the 50s how much more power they were getting.

From Eisenhower’s farewell speech in 1961:

“This conjunction of an immense military establishment and a large arms industry is new in the American experience. The total influence – economic, political, even spiritual – is felt in every city, every Statehouse, every office of the Federal government. We recognize the imperative need for this development. Yet we must not fail to comprehend its grave implications. Our toil, resources and livelihood are all involved; so is the very structure of our society.

In the councils of government, we must guard against the acquisition of unwarranted influence, whether sought or unsought, by the military-industrial complex. The potential for the disastrous rise of misplaced power exists and will persist.

We must never let the weight of this combination endanger our liberties or democratic processes. We should take nothing for granted. Only an alert and knowledgeable citizenry can compel the proper meshing of the huge industrial and military machinery of defense with our peaceful methods and goals, so that security and liberty may prosper together. “

Too bad his words weren’t heeded. What he feared is exactly what we have now. A military industry complex that now has unwarrented influence over our foreign and domestic policy, as well as our economy.