Fouad Ajami, a senior fellow at Stanford University’s Hoover Institution, is the author of “The Syrian Rebellion” and “Dream Palace of the Arabs: A Generation's Odyssey.”

It is rarely a good idea to draw maps in a hurry. But that is what colonial cartographers did in the Arab world after the First World War, and the borders they painted were superimposed on old tribal and religious attachments that long predated the new states.

Today, the folly of those lines is made clear, as Syria’s war threatens not just its territorial unity but that of its neighbors as well.

Alas, it was perhaps optimistic to ever imagine that the fighting between Syria’s Alawite regime and the Sunni-led rebellion would remain within the country’s borders. Syria is at once the pivot and a mirror of the Fertile Crescent, and its sectarian and ethnic fissures reproduce themselves in neighboring Arab states. As an oddly passive President Obama ponders what he might do in Syria — and whether to do anything at all — he should be less preoccupied with red lines of his own making than with the blurring of the lines drawn in Arab sands decades ago.

On the map, Tripoli, on the Mediterranean Sea, lies within the borders of Lebanon and is the country’s second-largest city. But Tripoli, staunchly Sunni, with an Alawite minority, has always been within the orbit of the Syrian city of Homs. So it is no mystery that a deadly conflict now rages in Tripoli between Sunni and Alawite neighborhoods, rendering the place ungovernable. Sunni jihadists and preachers see the Syrian struggle as their own, an opportunity to evict the Alawites from their midst and to restore Sunni primacy.

Look to Iraq, on Syria’s eastern border, for the region’s quintessential artificial entity. Today, the government in Baghdad, Shiite-led for the first time in a millennium, sides with the Alawite dictatorship in Damascus. But in western Iraq, the Sunni strongholds of Anbar province and Mosul have been stirred up by the Syrian rebellion. The same tribes straddle the border between the two countries. Smugglers and traders, and now Sunni warriors, pay that border no heed.

The American war upended the order of things in Iraq; the Sunni minority lost out to the Shiites and bristled under that change of fortunes. The Syrian rebellion, a Sunni upheaval against an Alawite minority, has been a boon to the Sunnis of Iraq. The Sunnis have bottomless grievances against the government of Prime Minister Nouri al-Maliki. To them, Maliki, who spent a quarter-century exiled in Syria and Iran, is an agent of the Iranian theocracy. So even though the regime in Syria did its best to subvert the new order in Baghdad — between 2003 and 2009, Syria was the transit point for jihadists converging on Iraq to fight the Americans and the Shiites — the Maliki government, with oil money, and anchored in the power of the (Shiite) Dawa party, is throwing a lifeline to the Syrian dictator.

The Shiite appetite in Iraq has grown with the eating. Anti-terrorism laws and the provisions of de-Baathification have been unleashed on the Sunnis, and the forces of order have become instruments of the Maliki government. Thousands languish in prison on spurious charges, and protests have broken out in Sunni cities. The Syrian conflict has added fuel to the fire. If the Sunnis needed proof that the Shiite coalition in the region (comprising Iran, the Iraqi state, the Alawite regime in Damascus and Hezbollah in Lebanon) is hell-bent on robbing them of their historic place in Iraq, their government’s tilt toward Bashar al-Assad provided it.

It was a matter of time before these millennial conflicts were given new life by the Syrian civil war, which has acquired the passion of a religious calling. So Shiite warriors from Iraq and Lebanon flock to Syria today, they tell us, to protect the shrine of Sayyida Zeinab in the eastern suburbs of Damascus. It is easy work for Hasan Nasrallah, the secretary general of Hezbollah, to dispatch young foot soldiers to Damascus and drape his support for the Syrian dictator in the garb of religious duty.

Terrorist groups, Nasrallah said on April 30, had threatened to overrun and destroy the shrine. “If such a crime were to take place, it will carry with it grave consequences,” he warned. “Countries supporting these groups will be held responsible for this crime.”

Nasrallah is not a subtle man. He proclaimed nothing less than a sectarian war over Syria: “Syria has real friends in the region, and the world will not allow Syria to fall into the hands of America, Israel and the takfiri groups,” or militant Islamists. Nasrallah, very much in the tradition of Maliki in Baghdad, offended the Sunnis in his own country. Sunni preachers in Beirut, Sidon and Tripoli have called on their own to rise to the defense of the Syrian rebellion.

The schism over Syria was given away in a Pew survey released May 1 that found 91 percent of Lebanon’s Shiites had a favorable opinion of Assad, and 8 percent held an unfavorable one. The results among the country’s Sunnis were the reverse: 7 percent favorable, 92 percent unfavorable. Such estrangement in a small, claustrophobic country!

Syria’s war plays out differently among its neighbors. Jordan, through no choice of its own, is caught up in the struggle as southern Syria, for all practical purposes, spills over its border. An estimated 500,000 Syrians have made their way into Jordan — almost a staggering 10 percent of that country’s population. Jordan is overwhelmingly Sunni, so it has been spared the virulence of the vendettas blowing through Iraq and Lebanon. But it has its own fault line — between a secular monarchy and a strong Muslim Brotherhood. The Brotherhood is invested in the success of the rebellion in Syria, and the monarchy is on edge. It can’t close its border in the face of the Syrians, and it struggles to cope with a huge economic burden amid its own scarcities. It waits for deliverance — help from the Gulf Arabs and from the United States — and prays for an end to this war from hell.

Israel is, of course, a Syrian neighbor apart. Wisely, it initially kept a policy of benign yet watchful neglect of this fight. There was no love lost for the Syrian dictatorship but no faith that the rebels would make better neighbors if and when they came to power. On the one hand, the dictatorship, under Assad and his father before him, had kept the peace on the Israeli-Syrian border. But the Syrians had also stoked tensions on the Lebanese-Israeli frontier and had given Iran access to the Mediterranean, so perilously close to Israel. It was the better part of wisdom to steer clear of Syria’s fire.

But alleged Israeli airstrikes over Damascus in recent days have demonstrated the limits of Israel’s patience. The targets were depots of Iranian missiles, meant to be delivered to Hezbollah. These missiles had a range of 200 miles and could carry half-ton warheads. The Israelis made good on their “red line.” They would not permit Hezbollah that kind of power over their security.

Even with all this instability, I don’t believe that the borders of the Fertile Crescent will be erased. Western Iraq will not secede and join Syria, nor will Tripoli slip into Syria. But a Syria ruled by a Sunni majority would rewrite the rules of the region’s politics.

It could put an end to the militarization of Syrian society that has wrecked that country. Free of despotism, the Syrian middle class might erect the foundations of a more open and merciful nation. Syria is a land of merchants and commerce, and therein lies the hope that a better country could emerge from this ruin.

Lebanon, too, would be given a chance at normalcy. The power of Hezbollah in that country has derived to a great extent from the power of the Syrian dictatorship. If Syria is transformed, Lebanon must change as well, and the power of Hezbollah could be cut down to size. Utopia will not visit the region after the fall of the Syrian tyranny, but there is no denying that better politics may take hold in Syria and in its immediate neighborhood.

The remarkable thing about this drawn-out fight, now entering its third year, is the passivity of the United States. A region of traditional American influence has been left to fend for itself.

Of course, these sectarian enmities do not lend themselves to an outsider’s touch. Nor did Obama call up these furies; they cannot be laid at his doorstep. But the unwillingness of his administration to make a clean break with Assad helped radicalize the Syrian rebellion. The landscape would have been altered by American help. A no-fly zone near the border with Turkey could have sheltered and aided the rebels. An early decision to arm the rebellion would have leveled the killing field. Four of the president’s principal foreign policy advisers from his first term advocated giving weapons to the rebels — Secretary of State Hillary Rodham Clinton, Defense Secretary Leon Panetta, CIA Director David Petraeusand the chairman of the Joint Chiefs of Staff, Gen. Martin Dempsey. But the president overrode them, his caution of no help in a conflict of such virulence.

Under the gaze of the world, Obama instead drew a red line on the use of chemical weapons and warned that his calculus would change if these weapons were used or moved around. He thus placed his credibility in the hands of the Syrian dictator and, in the midst of a storm of his own making, fell back on lawyerly distinctions.

A Greater Middle East, an Islamic world, used to American campaigns of rescue — Kuwait in 1991, Bosnia in 1995, Kosovo in 1999, Afghanistan in 2001, Iraq in 2003, Libya in 2011 — is now witnessing the ebb of American power and responsibility. Obama has held his fire in the face of great slaughter, and truth be known, congressional and popular opinion have given him a pass. America has wearied of Middle Eastern wars.

Syrian rebels sure that the American cavalry would turn up after this or that massacre have been bitterly disappointed. It’s the tragic luck of the Syrians that their rebellion has happened on the watch of an American president who has made a fetish of caution, who has seen the risks of action and overlooked the consequences of abdication.

His prior Outlook essays include:

Why is the Arab world so easily offended?

Five myths about the Arab Spring

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