BEIRUT, Lebanon — American-backed forces have begun an assault on Raqqa, the Islamic State's hub in northern Syria, and signs are that they could capture the long-sought target with relative ease. Yet the militant group's commanders, who have already withdrawn their toughest forces from the city, and most everyone else in Syria's multifaceted war are looking ahead to an even more decisive battle in the south.
There, a complex confrontation is unfolding, with far more geopolitical import and risk. The Islamic State is expected to make its last stand not in Raqqa but in an area that encompasses the borders with Iraq and Jordan and much of Syria's modest oil reserves, making it important in stabilizing Syria and influencing its neighboring countries.
Whoever lays claim to the sparsely populated area in this 21st-century version of the Great Game not only will take credit for seizing what is likely to be the Islamic State's last patch of a territorial caliphate in Syria, but also will play an important role in determining Syria's future and the postwar dynamics of the region.
With the stakes so high, the United States, Iran and Russia are all scrambling for advantage. They are building up their forces and proxy fighters and, increasingly, engaging in inflammatory clashes that threaten to escalate into a larger conflict.
On Thursday, an American pilot shot down an Iranian-made drone as big as an American Predator, which had fired on American-backed Syrian fighters and American Special Forces advisers.
All have their eyes on the province of Deir al-Zour, where Islamic State forces surround an estimated 200,000 people in a government-held section of the provincial capital of the same name.
The contested area also includes desert regions farther south that include several border crossings, including the critical highway connecting Damascus and Baghdad — coveted by Iran as a land route to Lebanon and its ally, the Shiite militia Hezbollah.
But what is really at stake are even larger issues. Will the Syrian government re-establish control of the country all the way to its eastern borders? Will the desert straddling the Syrian-Iraqi border remain a no man's land ripe for militant control? If not, who will dominate there — forces aligned with Iran, Russia or the United States? Which Syrian factions will wield the most influence?
The moment is a ”major crossroads" in the conflict, said Kamel Wazne, who studies Hezbollah, the United States and the Middle East and teaches at the American University of Beirut.
The Americans want to prevent the establishment of a ”Shiite crescent" of influence from Iran to Lebanon, Mr. Wazne said, and to maintain ”a piece of what is taking place in Syria."
”They will not allow the Iranians and those they support to have a victory at the expense of the Americans in the whole region," he added.
That, Mr. Wazne said, puts the United States at loggerheads with the pro-government alliance in Syria, especially Hezbollah and Iran. With President Trump and his newly emboldened Sunni partners in Saudi Arabia and the Persian Gulf turning up the heat against Iran, Tehran and its allies will be determined, he said, to show they cannot be intimidated.
”They can drive the confrontation," he said. ”This camp is actually more determined to carry through with the fight, in their minds the ultimate confrontation."
Indeed, on Friday, pro-government forces struck what could be a major blow to American plans, making a surprise advance to the Iraqi border that cut off American-backed troops, blocking their way to the front against Islamic State in Deir al-Zour.
The lineup of combatants is dizzying. Moving east from the heart of Syria is the alliance backing President Bashar al-Assad, consisting of the Syrian Army and Iran-backed militias, supported by Russian air power and Iranian advisers. Some reports even suggest that Russian advisers are active on the battlefield.
Moving north from the Jordanian border are Syrian rebels who have long fought the government but are now being trained to fight the Islamic State, also known as ISIS or ISIL, by American, British and Norwegian forces. They have a garrison near the Jordanian and Iraqi borders and the Baghdad highway, and receive air support from the United States-led, anti-ISIS coalition.
Then there is a different American-backed force, the one attacking the Islamic State in Raqqa. There are signs that to take Raqqa without an all-out fight, they will let more Islamic State fighters escape to the south. And their leaders have voiced ambitions to follow the fight south to Deir al-Zour.
That is a problematic prospect because the force, known as the Syrian Democratic Forces, is led by Syrian Kurdish militias that have established a semiautonomous Kurdish zone farther north and are distrusted by many Arabs. American officials have sought to allay those concerns by noting the force is half Arab and saying it will hand over retaken areas to civilian local councils.
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Finally, in Iraq, Iran-backed Iraqi Shiite militias have pushed west to the border with Syria.
With all these forces on a collision course, several recent escalations have raised fears of a direct confrontation between the United States and Iran, or even Russia.
While saying it does not seek confrontation with the Syrian government or its allies, the United States has begun deliberately bombing them, something it had not done before in the war, hitting Iranian-backed militias it deemed too close to Tanf, the site of the American and rebel garrison.
But the pro-government alliance has not backed down. It has denounced the American presence as illegal and continued to confront the allied forces, culminating in Thursday's drone strike.
That episode showed that Iranian advisers, or perhaps even Iranian proxies like Hezbollah, are operating full-size drones in Syria and are willing to risk clashes with the United States.
Looming over everything is the question of how far Russia is prepared to back Iran — a tactical ally but one with which it differs strategically on major issues like Israel and the United States. American generals believe that the answer is ”not far." Secretary of Defense James Mattis even said Russia had tried to persuade the Iranian-backed forces not to approach the American base.
But diplomats in Beirut say Russia may be saying the opposite to its allies in Syria, and may see little reason to halt what could be an awkward test for the United States, which insists it does not want to get more deeply involved in Syria. Will it risk war to protect a small base?
One diplomat who was briefed on the Russian position said Moscow wanted to see that the American-backed rebels left the area.
The rising tensions reflect increased strains in the region. The Trump administration has vowed to push back against Iran. Israel, saying it will not tolerate a Hezbollah presence abutting its territory, has been striking pro-government forces in the south.
Saudi Arabia, emboldened by Mr. Trump, has increased the pressure, isolating Qatar in part for its cordial relations with Iran. Islamic State attacks in Tehran on Wednesday further inflamed sentiments, and Iran said Saudi Arabia, Israel and the United States were ultimately to blame.
Now, into that volatile mix, toss a battle that will reveal the unsustainable contradictions of the fight against the Islamic State as the forces arrayed against it jockey against one another.
As always, the Deir al-Zour arena is a rat's nest of contradicting alliances. As they converge, something will have to give.
For instance, the United States is working indirectly in Iraq with Iranian-backed Iraqi Shiite militias — some of the very ones that it has fired on in Syria's southeastern desert.
Islamic State fighters are besieging Syrian government forces in the provincial capital, Deir al-Zour, so by routing them, the United States would be helping Mr. Assad, whom it opposes.
Mr. Assad says that Syria's sovereignty is paramount and that he will retake all lost territory, but he cannot do so without the help of Iran and Russia, which exercise a striking degree of autonomy and authority in Syria. Nor can he prevent the United States from operating there.
The Kurdish militias have so far hedged, neither directly confronting the pro-government forces nor openly collaborating with them, to avoid alienating Arab opposition allies. But they may have to choose, as the forces come into contact.
The dangers were underscored by a Syrian member of a Hezbollah-trained militia that took part in the recent drone attack on the American garrison. He said Shiite militiamen and technicians from Iran were monitoring the base with a drone, which American planes then destroyed on the ground. Five people, including two Iranians, were injured.
A second drone shot at the rebels and the Americans, said the fighter, who spoke on the condition of anonymity. ”This is a message to the Americans," he said. ”They can't tell us there are places we're not allowed to approach."