Imagine that the twin towers still dominated the Manhattan skyline. Imagine that the Pentagon’s western facade had remained intact. Imagine that there was no reason to build a memorial in Shanksville, Pa. And imagine that the numbers 9 and 11 meant nothing more than an emergency telephone call.

The world changed on Sept. 11, 2001, that much is clear. But how much, and how radically?

Historians and novelists love constructing “what if” scenarios: What if Hitler had won World War II? What if the Confederacy had prevailed in the Civil War? What if a Chinese sailor, rather than Columbus, had discovered America?

Such debates are a simple game, at worst. But at its best, counterfactual history helps us distinguish what is truly constant in our time from what is merely contingent, what is enduring from what is fleeting.

So, with that in mind: What if the al-Qaeda attacks on the United States on Sept. 11, 2001, had never happened? What if the plan to hijack airplanes and crash them into the World Trade Center, the Pentagon and possibly the Capitol had been called off or thwarted, or never even developed?

Of course, Osama bin Laden and his allies might have carried out another catastrophic and shocking attack. Absent such an event, however, U.S. efforts against al-Qaeda might have continued in their more limited pre-9/11 forms: some drone strikes against al-Qaeda bases in Afghanistan and Sudan, pressure on the Taliban to expel bin Laden, international intelligence and law enforcement cooperation. We’d never have known the “war on terror” as an organizing principle of an entire presidential administration.

And Osama bin Laden would never have become a household name.

Without the catalyst of the attacks, Congress would not have undertaken the greatest reorganization of the national security bureaucracy since the Truman years, stitching together the Department of Homeland Security from nearly two dozen agencies. Airline travel would still have its annoyances, but massively intrusive security screening might not be one of them.

American foreign policy would be strikingly different and almost certainly less bellicose. In the early months of the Bush administration, U.S.-Chinese tensions were high, and many American neoconservatives were focused on the rise of China as a military “peer competitor.” If the World Trade Center and the Pentagon had never come under attack, U.S. defense policy probably would have concentrated more on the Pacific than on the Middle East. A direct confrontation with China would have been unlikely, but without a distracting war on terror, Washington would have focused much more on China’s military buildup, its deals with anti-American regimes, and cyber-attacks by Chinese hackers against the United States and its allies.

No Sept. 11 means no invasion of Afghanistan, and possibly no invasion of Iraq. At most, we might have seen covert actions and more cruise missile attacks, such as those the Clinton administration launched in 1998, against countries harboring bin Laden and his allies. And terms such as “IED” (improvised explosive device) and “TBI” (traumatic brain injury) would not have become the defining reality for a generation of American troops.

Without the wars in Iraq and Afghanistan, there would have been no effort to reorient the U.S. armed forces toward counterinsurgency operations. No 9/11, no COIN. America’s defense planners might have spent the first decade of the 21st century focusing on possible high-tech naval and air combat in Asia, rather than on policing and nation-building in occupied Muslim countries.

And the opening of an Islamic community center in Lower Manhattan would have been a nonevent.

Certainly, the Middle East would still have been a priority for Washington, and Iraq’s Saddam Hussein would have remained one of America’s chief enemies in the region. But if 9/11 had never occurred, the United States might have continued its post-Persian Gulf War policy of “containment” against Hussein’s regime to this day — through no-fly zones and intermittent bombing of the territory that Hussein controlled.

But it is possible that, even without 9/11, the Bush administration would have chosen to invade Iraq. The much-hyped threat of Iraqi weapons of mass destruction — which turned out not to exist — might have been enough for Washington to launch an invasion. Or the United States might have carried out plans, favored by many neoconservatives before 9/11, to back an Iraqi rebellion, a scheme opponents called the “Bay of Goats” scenario. If Hussein’s regime had fallen as a result of an American invasion or a domestic rebellion, then a bloody fracturing of the country along ethnic lines might have occurred regardless.

Without 9/11, Iran, another chief U.S. rival in the region, almost certainly would wield less influence than it does today. America’s wars in Iraq and Afghanistan removed two of Iran’s major enemies — Hussein, who had waged war against Iran, and the Taliban, a Sunni radical regime that despised Iran’s Shiite mullahs. And the destruction of Hussein’s Sunni-minority regime has allowed Iran to greatly increase its influence among Iraq’s Shiite majority.

And what of the Arab Spring? How did 9/11 affect this movement? It seems clear that the timing of the revolts in the Arab world has had more to do with the long-term decay of sclerotic dictatorships than with anything the jihadist movement has offered — in part because its atrocities against Muslims as well as against “crusaders” and “Jews” have disgusted many Muslims. Paradoxically, if Muslim radicalism had been less tainted by the mass murder of 9/11, its long-term goal of replacing Arab regimes with Muslim theocracies might have had more appeal among the region’s revolutionaries.

On the home front, American politics would have unfolded in vastly different ways. Assume that, with no 9/11 to use as a pretext to invade Iraq, the election of 2004 would have been a peacetime contest, centered largely on economic issues. It may seem minor compared with today’s economic difficulties, but recall how a “jobless recovery” followed the crash of the tech bubble, and the subsequent bubble in real estate and stocks took time to inflate. George W. Bush, like his father before him, might not have won reelection if the Democratic nominee had run a reprise of the 1992 Clinton campaign, with the message “it’s the economy, stupid.”

But that doesn’t mean America would have had a President John Kerry. Kerry secured the Democratic nomination in 2004 in large part because primary voters believed that his war record and national security expertise made the Democrats less vulnerable to the claim that they were soft on terrorism. Remember his acceptance speech’s opening line, which brought the crowd at the national convention in Boston to its feet? “I’m John Kerry, and I’m reporting for duty!”

Without the backdrop of the war on terror, a Vietnam War record might not have seemed to be a plus to Democratic primary voters, who are to the left of Democrats overall and the electorate in general. They might have chosen another candidate, one more in tune with new fundraising technologies, one focused on the populist economic issues that would have dominated the peacetime election.

President Howard Dean, anyone?

Whether Dean, Kerry or someone else, the winning Democrat in 2004 would almost certainly have run for reelection in 2008 (it is hard to imagine Hillary Rodham Clinton launching a primary challenge against a sitting president). That means Barack Obama might still be a senator from Illinois. There would have been no history-making first African American president, no birth-certificate controversy — and Obama could still be friends with his longtime pastor, the Rev. Jeremiah Wright.

If Bush, unable to run as a wartime president, had lost in 2004, he would not have had four more years to alienate Republican moderates with wars and deficits. In this parallel universe, that group of voters might have stayed with the GOP rather than defecting to the Democrats in great numbers in 2008 — and would have wielded more influence in contemporary politics than the tea party movement. And unlike Obama, a Democratic president in a world without 9/11 might have paid less attention to right-leaning independents and governed as more of a progressive.

And what of the U.S. economy in a world in which Mohamed Atta and his fellow terrorists never hijacked airplanes? If the United States had not invaded Afghanistan and Iraq, the national deficit and debt would be considerably lower today. According to a recent report by the Congressional Research Service, between September 2001 and March 2011, Congress appropriated $1.283 trillion for the wars, additional security measures and health care for veterans — with 63 percent of the total related to Iraq and 35 percent to Afghanistan. Economists Joseph Stiglitz and Linda Bilmes have estimated that the long-term cost of the wars, including veterans’ care, may exceed $3 trillion.

That’s a lot of money, no doubt. But as a contribution to the long-term budget deficit, the combined costs of the wars are lower than the costs of the Great Recession and related recovery measures. Recession-driven deficits, in turn, are lower than the long-term deficits caused by the Bush tax cuts.

Bottom line: Even without 9/11 and the subsequent wars, the United States still might be facing huge deficits — particularly if the Bush tax cuts had been enacted and if the stock and real estate bubbles had inflated and then popped, with disastrous effects for the global financial system and consumer demand.

Indeed, when historians write the actual — not counterfactual — history of our time, Sept. 11 might receive less attention than the crash of 2008, which produced the greatest global economic crisis since the Great Depression. The bubble that burst that year was a result of trends dating back to the 1970s, including American consumers taking on excessive debt to finance consumption, despite their stagnant incomes, and the low interest rates made possible by Japan and China, which recycled their huge trade surpluses into purchases of U.S. debt to keep their currencies artificially low and their exports competitive.

These macroeconomic imbalances would have made a global economic reckoning inevitable, with or without 9/11. As it happens, the costs to the world economy after the bubble burst, in lost wealth and slow growth, dwarf both the direct economic damage inflicted on Sept. 11, 2001 — $40 billion in insurance costs, temporary losses in the airline business and a stock market decline — and the indirect costs of the wars in Iraq and Afghanistan and of spending on homeland security. And its possible full costs, including the failure of Europe’s experiment in integration and the self-defeating moves by countries exporting unemployment to one other through protectionist policies, have yet to be tallied by historians.

The fall of Lehman Brothers has resulted in far more economic damage and greater long-run consequences than the fall of the twin towers. This is not to minimize the horror of 9/11, its tragic death toll or the costs of its aftermath, but to put them in perspective. Whether the attacks of Sept. 11 had taken place or not, the world almost certainly would have been devastated by weapons of mass destruction — not airplanes hijacked by jihadists, nor the imaginary atomic bombs and chemical weapons of Saddam Hussein, but explosive credit derivatives in the hands of the world’s bankers.

Michael Lind is policy director of the Economic Growth Program at the New America Foundation and the author of “The American Way of Strategy.”

Read “Five myths about 9/11” and “9/11 has become all about New York — with Washington and the Pentagon nearly forgotten.”

Read more from Outlook, friend us on Facebook, and follow us on Twitter.

The New America Foundation’s Michael Lind imagines an alternative universe — that isn’t so alternative