The first watershed of the 21st century occurred when a hijacked jetliner slammed into the World Trade Center in 2001. The second, equally significant, occurred when American forces sealed their occupation of Baghdad in 2003.

With the advent of the war on terror and, now, the toppling of Iraq, the United States suddenly has more power than it ever possessed, more Middle East and Central Asian bases than it ever dreamed possible, more financial burdens than it ever shouldered, and more points of security vulnerability than it has ever had.

The new world order that has emerged from the smoke of Baghdad is only slightly less dramatic than the one that followed the defeat of Nazi Germany and Imperial Japan. In 1945, the United States shared world power with, and was threatened by, the Soviet Union. In 2003, it shares world power with no conventional state and is threatened only by a handful of rogue states (chief menace: North Korea) and a handful of rogue terrorist organizations (chief peril: al-Qaida).

Look at the new landscape: The United States has swiftly acquired four new bases in the land it once considered the biggest threat to world stability (Iraq). Since the terrorist attacks of September 2001, the United States has planted bases in parts of what used to be known as the Soviet Empire, including three in former Warsaw Pact nations (Hungary, Romania and Bulgaria), two in what used to be republics of the USSR (Uzbekistan and Kyrgyzstan), and one in a nation known as a breeding ground of radical Islamic fundamentalism (Pakistan).

These bases are instruments of projecting American power and are footholds in a region that only two years ago posed a mortal threat to the security of the West. In just the flash of an eye — one in which the United Nations has vanished from significance — the entire architecture of global power has been altered.

“We’re in an entirely different place than we were a month ago,” says Eliot Cohen, a professor of strategic studies at Johns Hopkins University’s School of Advanced International Studies. “There are several remarkable things about this war — besides what we did in it. The war was conducted despite international opposition. It showed our sheer military power. And it changed the diplomatic world.”

American relations with France have been transformed, perhaps for years. There is a new sense of sobriety in American relations with Russia. So diminished is the United Nations that major European countries are now struggling, desperately and perhaps ultimately unsuccessfully, to find a new role for it, perhaps in coordinating humanitarian aid to Iraqis, perhaps in building new political institutions for Iraqis. When Secretary of State Colin L. Powell talked last week about peacekeepers, for example, he specifically cited NATO, not the U.N. Regime change in the Persian Gulf has forced a change in regime, or at least in role, on Turtle Bay.

But for all the talk of an American Empire, the current period is more of an American enigma. The deficit is careening toward record heights. American economic power is built on foreign debt. The Bush administration carried a big stick in Iraq, but, after a short flurry a week ago, shows little taste for brandishing it in Syria, Iran or North Korea. And though the United States spends more than the rest of the world combined on national security, there is no domestic consensus on what the nation’s role should be around the globe.

Right now the big debate is over the cost and character of the American occupation of Iraq. Pentagon officials speak of the need for American soldiers and aviators to remain in the country for 360 days — far less than the four years required in Germany and the seven in Japan. No one knows the cost of the Iraq reconstruction effort, though administration officials believe it could cost as much as $20 billion a year.

In a study he prepared before the war for the American Academy of Arts and Sciences, William D. Nordhaus, a Yale economist, concluded that the total bill could be between $100 billion and a little under $2 trillion. “Since then, I’ve seen little to change my mind,” he said this week. “We’re going to find that this is an expensive undertaking. We’re spending $400 billion a year managing our role in the world, probably close to $500 billion this year. Managing the security affairs of the world costs a lot of money.”

If this is empire, it is empire without enthusiasm. “(H)alf the glory of the Pax Britannica,” wrote James Morris, “was its far-flungness, spangling the seas with forts and islets as the power of some great stained-glass window issues from a thousand pretty parts.” Morris said there was a “high romance” to the British Empire.

No romance here. Britain luxuriated in the exotic nature of its charges. America is scared silly by the occupants of its. Then again, the British, contemplating the unknown in peoples and lands far away, were inspired by the notion of terra incognito. America, contemplating the threats from peoples and lands it hardly knows, is obsessed with terror incognito. Different world, different world empire.

David Shribman is a columnist for the Pittsburgh Post-Gazette.