A deadly U.S. airstrike on Syrian government-backed forces this week has increased fears that American forces sent to battle Islamic State are being dragged more and more into the country’s bloody 7-year-old civil war.

The Pentagon insisted Thursday that the strike, which reportedly killed as many 100 fighters loyal to Syrian President Bashar Assad, was carried out in self-defense, but it sparked a furious reaction from Damascus and from Mr. Assad’s allies in Russia and Iran. Russia demanded closed consultations at the U.N. Security Council on Thursday afternoon, and the Assad government accused Washington of seeking an illegal de facto partition of the country.

Private analysts say the roughly 2,000 American troops in Syria who had been working with local Kurdish and Arab allies battling Islamic State in eastern Syria risk getting caught in “mission creep” as Islamic State loses its foothold in the country but the chaotic civil war between Mr. Assad and a slew of rebel and terrorist groups shows no signs of ending.

“We risk getting into a rapidly escalating situation in which we have bitten off more than we can chew,” said Hal Brands, a former high-level Pentagon adviser who teaches at Johns Hopkins University’s School of Advanced International Studies.

A Pentagon spokeswoman said Wednesday’s airstrikes were authorized only after forces loyal to the Assad regime, moving in a battalion-sized formation with artillery, tanks, multiple-launch rocket systems and mortars, fired about 25 rounds very near the headquarters of the U.S.-backed Syrian Defense Forces, which is opposed by the Assad government and Islamic State, in eastern Syria.

Defense Secretary James Mattis was at pains Thursday to deny that the airstrikes signaled any attempt by the Trump administration to get more deeply involved in the Syrian conflict. He insisted to reporters at the Pentagon that the troops were acting “in self-defense.”

“We are not getting engaged in the Syrian civil war,” he said, adding that it was perplexing to him why the heavily armed government battalion would cross a well-established “deconfliction line” to shell an area where it was known that U.S. special operations forces were working with the SDF.

“We are there to fight ISIS. That’s what those troops were doing — coordinating strikes against ISIS,” Mr. Mattis said. “Why they chose to initiate this attack, you’ll have to ask them. We don’t know.”

But the incident underscores the growing risk of a clash — accidental or intentional — on a confusing battlefield where rebels fight Syrian government troops as Russia, the U.S., Iran, Turkey and the Kurds all have forces on the ground. Mr. Mattis acknowledged that he could neither confirm nor deny reports that Russian military contractors were among those killed in the U.S. counterstrike, and he wasn’t even sure of the exact composition of the foe.

“We know they were pro-regime forces,” he said, “Iranian, Assad, Russian, mercenaries, I can’t tell you.”

Anger in Syria, Russia

The Syrian government condemned the airstrikes as a “war crime,” and the Russian Defense Ministry said the incident exposed the true motivation behind the U.S. military mission in Syria.

The incident “shows that the United States’ illegal military presence in Syria is actually aimed at taking control of the country’s economic assets and not at fighting against [Islamic State],” the ministry said in a statement, noting the close proximity of the U.S. strikes to oil fields in eastern Syria.

The ministry claimed a team of Syrian troops and pro-Assad militias were carrying out an attack on a “terrorist group” that had been shelling Syrian government positions near the oil fields. During the exchange, the ministry said, Syrian forces suddenly came under mortar and artillery fire, followed by the “U.S.-led coalition’s helicopters.”

But the statement also said the pro-Assad forces had failed to inform Russian military advisers about their operations, suggesting that communications might have avoided the clash.

The presence of U.S. forces in Syria, begun under President Obama and expanded under President Trump, operates under a legal cloud. Some critics in Congress say Mr. Obama and Mr. Trump have gone far beyond the original authorization to fight terrorism related to the 9/11 attacks to justify the Syria mission. The Pentagon’s recent assertions that U.S. forces will remain in the eastern Syrian enclave to ensure Islamic State cannot reorganize, they say, points to an even more open-ended military commitment.

“While I am grateful that no U.S. or coalition members were harmed in the attack, I am gravely concerned that the Trump administration is purposefully stumbling into a broader conflict, without a vote of Congress or clear objectives,” Sen. Tim Kaine, Virginia Democrat, said at a Senate Armed Forces Committee hearing Thursday.

Mr. Brands told The Times that, “to the extent that the recent strikes were taken to defend U.S. forces and partner forces, they are both appropriate and justified.”

“[But] at the same time, this is an escalation, and it testifies to some underlying dynamics at work,” he said. “As ISIS is defeated, the other forces in Syria are being drawn into closer proximity with each other.

“The major danger for the U.S. in Syria is that we may have a significant ends-means mismatch in our policy,” Mr. Brands added.

He said the Trump administration has “an incredibly optimistic set of goals, including promoting a political transition away from Assad and pushing back against Iranian influence.”

State Department spokeswoman Heather Nauert defended the airstrikes and insisted that the military mission was critical to an eventual diplomatic end to the civil war being overseen by the United Nations.

“We are there also to stabilize the country so that the country, hopefully, can get through the Geneva process and to have elections and decide what they want to do with its future,” she said.

The Pentagon revealed last month that about 2,000 American troops are deployed inside Syria — roughly four times the number previously disclosed. Although some of the troops are believed to be directly fighting Islamic State elements still active in the war zone, officials have said troops are staying to train local forces in what is still legally considered sovereign Syrian territory.

Avoiding conflict

Hossam Abouzahr, deputy director at the Atlantic Council’s Rafik Hariri Center for the Middle East, said this week’s developments shouldn’t be read as “an indication that we’re going to see an increase in U.S. bombing of Syrian military assets.”

“But it is an indication that we’re willing to defend the demarcation line that we’ve set up along the Euphrates River between U.S.-backed Syrian opposition forces and Syrian government forces.”

The task of sorting out the conflict got only more difficult with a separate invasion launched two weeks ago by Turkey — technically an American military ally — to clear out a U.S.-backed Kurdish militia from areas along the border.

While Secretary of State Rex W. Tillerson and National Security Adviser H.R. McMaster are slated to discuss the situation during a visit to Turkey early next week, Turkish officials have said they are also planning a summit in Istanbul with Russian and Iranian leaders to discuss peace efforts for Syria.

Mr. Abouzahr said he believes a sharp escalation of the U.S. role is unlikely as long as the truce line with Russia continues to work. “I don’t think Russia wants a war,” he said.

But this week’s incidents show that the Pentagon may not control the narrative.

“The issue here is that if we don’t have a clear strategy of how to proceed in Syria, we could be drawn into a conflict with Russia on Russia’s terms,” said Hassan Mneimneh, a scholar at the Middle East Institute in Washington.

The Russians, he said, “have an endgame, and we do not. We’re just reacting.”

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