Unless you can remember life before World War I, the United States you know has always towered over the rest of the world. Since 1920 or so, the U.S. has been the world’s economic engine, its productivity expanding steadily and consistently leading the way in technology. This nation’s military is the largest and most powerful in world history. When World War I erupted in 1914, it was the U.S. joining the fray in 1917 that tipped the scales to victory for England and France. In 1941, the U.S. again, with its manufacturing prowess and manpower, turned aside Nazi and Japanese aggression in Europe and the Pacific. After that war, the Soviet Union tried to match the U.S. in competing for spheres of influence. But it, too, could not keep pace.

And the U.S. has been a moral and humanitarian force in the world.

But what if the so-called American Century is over? Booming economies in China and India were eclipsing the U.S., even before the current financial crisis. That fading prosperity has implications for the U.S. if it seeks to influence world affairs.

The country may learn a lesson from Great Britain. Through the late 19th and early 20th centuries, “the sun never set on the British Empire,” as the saying went. The Brits colonized or had leverage over India, Burma, Indonesia, Singapore, Hong Kong, Pakistan, Afghanistan, Iraq, Persia (modern-day Iran), Malaya, Palestine (modern-day Israel), Ireland and many African nations. Yet after World War II, Great Britain was reduced to food rationing.

Stephen Miller, a history professor at the University of Maine with a specialty in modern British history, said that nation struggled well into the 1960s with its new, post-empire identity, with some tragic consequences. Just before World War II, the Labor Party took power, and leaned away from empire-building.

After the war, Miller said, the British government saw the wisdom of granting independence to India and Palestine. But when the Conservative Party regained power in 1950, under former Prime Minister Winston Churchill and later under Anthony Eden, retaining British holdings around the globe was again a priority. That resulted in “some extremely bloody wars,” Miller said, in Africa and Southeast Asia.

By the 1960s, the British recognized their empire days were over, he said.

If the U.S. “informal empire,” as Miller describes it, is in decline, policy should reflect that diminished role. Wars of choice should be a thing of the past. Shrugging off trade deficits and a sinking dollar as temporary should be replaced with realistic plans to reverse or at least stall these trends.

And above all, deficit spending — largely financed by China — must end.

Today, Great Britain remains a player in world affairs, but in a limited way. Though Prime Minister Tony Blair supported the U.S. invasion of Iraq, the Brits sent just 40,000 troops, and by early 2007 had reduced it to 7,100.

Great Britain’s focus today is on its own prosperity within the context of the European Union. That more modest self-image is a better model for the U.S. than that of strutting superpower.