Colonialism just ain't what it used to be
1891, British armed forces stationed in (undivided) India: 90,666
Population of undivided India: 287, 223, 431
Number of Indians per British soldier: 3,168
2006, American troops stationed in Iraq: c. 150,000
Population of Iraq: 26,074,906
Number of Iraqis per American soldier: 174
More here. The ratio of Indians to soldiers of British origin was even higher under the old British East India Company. Furthermore, even though the British East India Company, and later the British Crown, unified and controlled and radically changed the government of India, British civil service in India "had a maximum strength of little more than a thousand," according to Niall Ferguson's colorful book Empire.
It's no longer our highly educated and culturally unified mercenaries taking sides in wars between badly divided and largely illiterate native polities, as during the colonial era. National sympathies, stemming mainly from ties of a written language and shared religion, now unify millions of people at a time into cohesive, educated, and highly motivated political blocs that we try to control at our peril. The sophisticated communications and financial networks such nations set up (even if they are stateless) cannot be disrupted for long. A small subset of such megagroups who have particularly strong views can, on the other hand, now severly disrupt traditional occupation and traditional government.
It's not now and never was a problem of "not enough troops on the ground." We are already deploying far more troops in Iraq than is historically normal for a successful overseas occupation. If we continue to try to play Empire, or even just world policeman, it will destroy us.