Shortly after the end of World War I, the French and British prime ministers took a break from the hard business of redrawing the map of Europe to discuss the easier matter of where frontiers would run in the newly conquered Middle East.

Two years earlier, in 1916, the two allies had agreed on their respective zones of influence in a secret pact—known as the Sykes-Picot agreement—for divvying up the region. But now the Ottoman Empire lay defeated, and the United Kingdom, having done most of the fighting against the Turks, felt that it had earned a juicier reward.

“Tell me what you want,” France’s Georges Clemenceau said to Britain’s David Lloyd George as they strolled in the French embassy in London.

“I want Mosul,” the British prime minister replied.

“You shall have it. Anything else?” Clemenceau asked.

In a few seconds, it was done. The huge Ottoman imperial province of Mosul, home to Sunni Arabs and Kurds and to plentiful oil, ended up as part of the newly created country of Iraq, not the newly created country of Syria.

The Ottomans ran a multilingual, multireligious empire, ruled by a sultan who also bore the title of caliph—commander of all the world’s Muslims. Having joined the losing side in the Great War, however, the Ottomans saw their empire summarily dismantled by European statesmen who knew little about the region’s people, geography and customs.

The resulting Middle Eastern states were often artificial creations, sometimes with implausibly straight lines for borders. They have kept going since then, by and large, remaining within their colonial-era frontiers despite repeated attempts at pan-Arab unification.

The built-in imbalances in some of these newly carved-out states—particularly Syria and Iraq—spawned brutal dictatorships that succeeded for decades in suppressing restive majorities and perpetuating the rule of minority groups.

But now it may all be coming to an end. Syria and Iraq have effectively ceased to function as states. Large parts of both countries lie beyond central government control, and the very meaning of Syrian and Iraqi nationhood has been hollowed out by the dominance of sectarian and ethnic identities.

The rise of Islamic State is the direct result of this meltdown. The Sunni extremist group’s leader, Abu Bakr al-Baghdadi, has proclaimed himself the new caliph and vowed to erase the shame of the “Sykes-Picot conspiracy.” After his men surged from their stronghold in Syria last summer and captured Mosul, now one of Iraq’s largest cities, he promised to destroy the old borders. In that offensive, one of the first actions taken by ISIS (as his group is also known) was to blow up the customs checkpoints between Syria and Iraq.

“What we are witnessing is the demise of the post-Ottoman order, the demise of the legitimate states,” says Francis Ricciardone, a former U.S. ambassador to Turkey and Egypt who is now at the Atlantic Council, a Washington think tank. “ISIS is a piece of that, and it is filling in a vacuum of the collapse of that order.”

In the mayhem now engulfing the Middle East, it is mostly the countries created a century ago by European colonialists that are coming apart. In the region’s more “natural” nations, a much stronger sense of shared history and tradition has, so far, prevented a similar implosion.

“Much of the conflict in the Middle East is the result of insecurity of contrived states,” says Husain Haqqani, an author and a former Pakistani ambassador to the U.S. “Contrived states need state ideologies to make up for lack of history and often flex muscles against their own people or against neighbors to consolidate their identity.”

In Egypt, with its millennial history and strong sense of identity, almost nobody questioned the country’s basic “Egyptian-ness” throughout the upheaval that has followed President Hosni Mubarak’s ouster in a 2011 revolution. As a result, most of Egypt’s institutions have survived the turbulence relatively intact, and violence has stopped well short of outright civil war.

Turkey and Iran—both of them, in bygone eras, the center of vast empires—have also gone largely unscathed in recent years, even though both have large ethnic minorities of their own, including Arabs and Kurds.

The Middle East’s “contrived” countries weren’t necessarily doomed to failure, and some of them—notably Jordan—aren’t collapsing, at least not yet. The world, after all, is full of multiethnic and multiconfessional states that are successful and prosperous, from Switzerland to Singapore to the U.S., which remains a relative newcomer as a nation compared with, say, Iran.

In all these places, a social compact—usually based on good governance and economic opportunity—often makes ethnic and religious diversity a source of strength, not an engine of instability. In the Middle East, by contrast, “in the cases where the wheels have come off, there was not good governance—there was in fact execrable governance,” says Mr. Ricciardone.

A century ago, many hoped that Syria and Iraq, too, would follow Switzerland’s path. At the time, President Woodrow Wilson sent a commission to the Middle East to explore what new nations should rise from the wreckage of the Ottoman Empire.

Under Ottoman rule, neither Syria nor Iraq existed as separate entities. Three Ottoman provinces—Baghdad, Basra and Mosul—roughly corresponded to today’s Iraq. Four others—Damascus, Beirut, Aleppo and Deir ez-Zor—included today’s Syria, Lebanon and much of Jordan and Palestine, as well as a large strip of southern Turkey. All were populated by a hodgepodge of communities—Sunni and Shiite Arabs, Kurds, Turkomans and Christians in Iraq, and in Syria, all these groups as well as Alawites and Druse.

President Wilson’s commissioners, Henry King and Charles Crane, reported back their findings in August 1919. In Europe at the time, the breakup of the Austro-Hungarian and Russian empires was leading to the birth of new ethnic-based nation-states. But the U.S. officials had different ideas: They advised Wilson to ignore the Middle East’s ethnic and religious differences.

What is now Iraq, they suggested, should stay united because “the wisdom of a united country needs no argument in the case of Mesopotamia.” They also argued for a “greater Syria”—an area that would have included today’s Lebanon, Jordan, Israel and the Palestinian territories.

The end of Ottoman rule, King and Crane argued, “gives a great opportunity—not likely to return—to build…a Near East State on the modern basis of full religious liberty, deliberately including various religious faiths, and especially guarding the rights of minorities.” The locals, they added, “ought to do far better under a state on modern lines” than under Ottoman rule.

The hopes of the Americans didn’t pan out.

In Syria, the French colonial authorities—faced with a hostile Sunni majority—courted favor with the Alawites, a minority offshoot of Shiite Islam that had suffered discrimination under Ottoman rule. The French even briefly created a separate Alawite state on what is now Syria’s Mediterranean coast and heavily recruited Alawites into the new armed forces.

In Iraq, where Shiites make up the majority, the British administrators—faced with a Shiite revolt soon after their occupation began—played a similar game. The new administration disproportionately relied on the Sunni Arab minority, which had prospered under the Ottomans and now rallied around the new Sunni king of Iraq, whom Britain had imported from newly independent Hijaz, a former Ottoman province since conquered by Saudi Arabia.

Those decisions helped to shape the future of Iraq and Syria once the colonial order was gone. The Assad family has ruled Syria since 1970; Saddam Hussein became president of Iraq in 1979. Notwithstanding their lofty rhetoric about a single Arab nation, both regimes turned their countries into places where the minority ruling communities (Alawites in Syria, Sunni Arabs in Iraq) were decidedly more equal than others.

Attempts by the Sunni majority in Syria or the Shiite majority in Iraq to challenge these harshly authoritarian orders were put down without mercy. In 1982, the Syrian regime bulldozed the largely Sunni city of Hama after an Islamist revolt, and Saddam unleashed his wrath to crush a Shiite uprising in southern Iraq after the Gulf War in 1991.

In Syria today, many Alawites are backing President Bashar al-Assad against largely Sunni rebels out of fear that the regime’s collapse could wipe out their entire community—a threat reinforced by Islamic State, whose Sunni extremists offer Alawites and mainstream Shiites a stark choice between conversion and death.

In Iraq, the Shiite-dominated governments that have ruled since the U.S. invasion in 2003 have turned the tables on the country’s former rulers by discriminating against the minority Sunnis. As a result, Islamic State managed to seize Sunni parts of Iraq last year largely unopposed because the group was often seen by the locals as a lesser evil.

“It’s not just the territorial boundaries that are an issue—it’s the map of governance that was contrived by Europe,” says Vali Nasr, dean of the School of Advanced International Studies at the Johns Hopkins University and a former State Department adviser. “Colonial powers within the states created colonial administrations that educated, recruited and empowered minorities. When they left, they left the power in the hands of those minorities—they left the dictatorship of the minorities.”

“Power was so out of alignment in Iraq, Syria and many of these countries, and there is no proper formula of how to make this right. The winners don’t want to share, the losers don’t want to give up power,” Dr. Nasr added. “The Middle East is going through a period of big turmoil, after which it will end up with a very different political configuration and perhaps also a different territorial configuration.”

But how much appetite is there in the Middle East to change these territorial configurations? And if they were changed, what might a new map of the region look like?

One obvious possibility involves the Kurds, whose desire to win an independent state in what is now eastern Turkey and northern Iraq was endorsed by the short-lived Treaty of Sèvres, a 1920 pact among the Western allies and the Ottomans. That treaty was promptly repudiated by Turkish nationalists led by Mustafa Kemal Atatürk, the founder of the modern Turkish state. Until recently, in fact, Turkey has denied the very existence of a separate Kurdish ethnicity.

The Kurds, who live scattered across Iraq, Turkey, Syria and Iran, have already enjoyed decades of virtual independence under an autonomous government in northern Iraq—the mountainous part of what was once the Ottoman province of Mosul. They have now established three autonomous “cantons” in northern Syria.

“I’d be surprised if, in 20 years, there won’t be a country called Kurdistan,” said Karim Sadjapour, a Middle East analyst at the Carnegie Endowment. “It already exists, de facto.”

With their separate language and culture, the Kurds in Iraq already control their borders and security, limiting entry by Arab Iraqis. As civil war has raged in Syria, Kurdish militias there have come to identify, by and large, with a different national project. “The other rebels fight for Syria, but we have our own Kurdistan, and that is what we care about,” said Farid Atti, an official with a secular Kurdish militia combating Islamic State near the town of Kobane, which is one of the three autonomous Kurdish “cantons” in Syria.

Beyond Kurdistan, however, the case for separate new nations becomes much less clear, despite the ethnic and sectarian horrors that torment the region today.

For one, no matter how artificial they originally were, the post-Ottoman states have proven surprisingly resilient. Consider Lebanon, a country of some 18 squabbling religious communities that survived a bloody, multi-sided civil war from 1975 to 1990 and has repeatedly defied predictions of its imminent demise. Despite—or perhaps because of—that strife-filled history, Lebanon remains an island of relative stability amid the current regional upheaval, even as it is being overwhelmed by more than a million Syrian refugees fleeing the chaos next door.

“The rulers of those countries that were formed along admittedly artificial borders initially have put plenty of effort into building a sense of nationalism. The question is how much it took?” says Michele Dunne, a former senior State Department official who is now a Middle East expert at the Carnegie Endowment. “It may not be as strong as in a country that had a sense of itself for centuries, but it still may be there.”

Indeed, even in battered and tattered Iraq and Syria, nationalist feelings remain very much alive. “If any country passed through what Iraq has passed through in the last 12 years, it would have been dismembered by now,” said Ayad Allawi, Iraq’s vice president and a former prime minister. “What kept the country going was the will of the people.”

In Syria, a 19-year-old student Mohammed Ali recently recalled the way that locals reacted to the arrival of Islamic State in his hometown of al-Boukamal, near the Iraqi border. As part of its campaign to erase colonial frontiers, the new rulers detached al-Boukamal from the Syrian province to which it belongs and incorporated it into Islamic State’s new “Province of Euphrates,” governed from the Iraqi city of Qaim.

At first, Mr. Ali said, the locals were excited by the destruction of the nearby border. “For 30 years, we have not been able to cross and visit our relatives on the other side,” Mr. Ali said. Since then, however, the mood has turned to patriotic backlash amid resentment of Iraqis flooding the area, lording over al-Boukamal and trucking “stolen” Syrian oil across the frontier. “We don’t want them here; we now want the border back,” he said.

Standing in the way of possible new partitions in the region is another set of issues: Where exactly would you draw the lines? And at what cost?

Despite the ethnic cleansing of recent years, Sunnis and Shiites still live together in many parts of Iraq, including Baghdad, and a great many Syrian Sunnis would still rather live in cities controlled by the Assad regime than in war-ravaged areas under rebel sway.

Mr. Allawi, the Iraqi vice president, points out that many of the country’s traditional tribal groups include both Shiites and Sunnis—and that many Iraqi families, especially in the larger cities, are mixed too. “You’d have to go through the bedrooms of people to separate the country,” he quips. And in Iraq as elsewhere, Sunnis, Shiites and Kurds are hardly unitary, consensus-driven groups; rivalries abound within them.

The only recent partition of an Arab country—the split of Sudan into the Arab north and the new, largely non-Arab Republic of South Sudan in 2011—doesn’t provide an encouraging precedent for would-be makers of new borders. South Sudan quickly slid into a civil war of its own that has killed tens of thousands and uprooted two million people.

“There is no alternative to replace the state system,” says Fawaz Gerges, who teaches Middle East studies at the London School of Economics. “Otherwise, you might replace one civil war with multiple civil wars, and that’s exactly what can happen in Syria or Iraq. This is a catastrophic cycle.”

Forging a new bottom-up social compact within the region’s existing borders—something likely to happen only after populations tire of endless wars—is the only way forward, says Stephen Hadley, who served as President George W. Bush’s national security adviser and now chairs the board of the U.S. Institute of Peace.

The real problem in the Middle East, he says, “is a collapse not of the borders but of what was happening inside the borders: governments that did not have a lot of legitimacy to start with and did not earn legitimacy with their people. You’re not going to solve these problems by redrawing the borders.”

Finding those solutions, Mr. Hadley acknowledges, won’t be easy.

“It may be past redeeming,” he says. “Getting out of this is going to be the work of a generation.”