After three weeks of steady fighting around the Islamic State's stronghold in Iraq, U.S. commanders say the group's defeat there is inevitable. But even once violence abates in Mosul, the broader fight against ISIS will be far from over.

In fact, many experts say, the campaign's next phase will likely get more complicated, require more American troops on the ground and result in a long-term commitment of U.S. forces to this region and others where ISIS is attempting to gain a foothold. Come January, the new commander in chief will confront a host of hard questions about how to carry on the two-and-a-half-year-old military mission started by President Barack Obama.

"This is going to be a preoccupation of whoever wins the U.S. election," said David Ucko, a counterterrorism expert who teaches at the National Defense University in Washington. 

Operation Inherent Resolve gets a new commander

Lt. Gen. Stephen Townsend became the seventh American general since 2003 to assume command of war operations in Iraq. (Daniel Woolfolk/Staff)

(Video by Daniel Woolfolk/Staff)

At this moment, there are two major issues.

In Iraq, U.S. officials expect that, once victory is declared in Mosul, ISIS will shift to guerrilla warfare and the fight there will continue as a counter-insurgency mission. The American military's potential role in such operations remains unclear.

In Syria, the U.S. and its allies are focused on the Islamic State’s self-proclaimed capital, Raqqa, an objective that will draw the American military deeper than ever into that country’s multi-sided civil war. On Sunday, Syrian Kurdish forces announced the startof its campaign to liberate the city, where an estimated 5,000 ISIS fighters are entrenched and upwards of 200,000 civilians remain trapped.

Defense Secretary Ash Carter on Sunday called the Raqqa operation a necessary step to "end the fiction of ISIL's caliphate." But while U.S. officials have signaled an urgent desire to oust ISIS the terror group from Syria, it's unknown what their near- or long-term strategy there will entail.

This much is clear: Once Mosul and Raqqa fall, and the Islamic State’s caliphate is destroyed, the U.S. military will almost certainly remain heavily engaged, conducting airstrikes, and providing tactical assistance and advisement for ground-level allies in addition to large-scale material support. The group's ideology has spread well beyond its rapidly diminishing territory inside Iraq and Syria. It's become a potent global threat, active in Afghanistan and in pockets of Europe, North Africa and Southeast Asia — at the backdoors of both China and Russia.

"Defeating ISIS doesn’t bring an end to the problem," said Anthony Cordesman, a former Pentagon official who is now a defense expert with the Center for Strategic and Budgetary Assessments.

"We can destroy extremist elements as a proto state. And that is very important," he said. "But this is not something that is going to have a quick, easy end. One way or other the foreign fighters who affiliated themselves with ISIS are still going to be around in significant numbers."

IRAQ BRACES FOR ANOTHER COUNTER-INSURGENCY FIGHT

As the battle for Mosul continues to progress, U.S. officials are optimistic. Coalition forces had advanced toward the city center, and intelligence assessments suggested there are about 3,000 ISIS fighters dug in there, compared to approximately 12,000 Iraqi and Kurdish troops surrounding them. The U.S. is providing daily airstrikes, artillery support and attack helicopters as well as vehicles, ammunition, food and other supplies.

If and when ISIS in Iraq no longer has outright control of any significant territory, the militants will likley dissolve into a shadowy insurgency.  "It is a strange echo of the situation we encountered about ten years ago. But at this point we are looking at a very different political context," Ucko said.

Battle for Mosul becomes fierce urban combat

Iraqi forces are struggling to secure gains they made over the past week on Mosul's eastern edge. This is transforming the fight against ISIS into close-quarters urban combat, spiking casualties among Iraq's troops and civilians. Iraq's military does not release official death tolls, but an American volunteer medic said he helped treat 44 casualties before he lost count. Hundreds of civilians slowly poured out of Mosul on foot during the fighting as well. Iraqi soldiers gathered them on street corners and loaded them on trucks set for camps for the displaced.

Politically, the U.S. is not willing to deploy large numbers of combat troops to Iraq. On the other hand, Washington has seen the fallout that can result from a complete military withdrawal. Ucko said the U.S. force level in Iraq, officially capped at about 5,200, is unlikely to decline after the battle of Mosul. The mission will shift to training, advising and assisting Iraqi forces on counter-insurgency operations.

"I think we will see a continuation of what some people might call an indirect approach," he said.

Ted Karasik, a Middle East expert with Gulf State Analytics, said the next president will confront questions about a long-term U.S. presence similar to the one Obama faced in 2011, when all operational forces were withdrawn.

"It’s easy to come up with the argument, once the U.S. feels comfortable that he Iraqis have the situation under control, that the U.S. can slowly wind down and leave. Then, of course, we’ll be back where we were 2012," Karasik said in a recent interview.

"We don’t want history to repeat itself, so the next administration will have to deal with that question."

A soldier with Iraq's elite counterterrorism force inspects a tunnel made by Islamic State militants in Bartella, Iraq, on Thursday, Oct. 27, 2016. The village is just east of Mosul.

BUT SYRIA MAKES IRAQ LOOK SIMPLE

The sense of urgency in Raqqa stems from new intelligence suggesting Islamic State leaders there are planning external attacks in the U.S. and Europe. But there’s no primary ally on the ground, as there is in Iraq.

"We have a huge tactical problem" in Raqqa, said Jim Jeffrey, a former ambassador to Iraq and Turkey who now works with the Washington Institute for Near East Policy. "Who is going to be the infantry?"

In Syria, the U.S. faces geopolitical crosswinds that make Iraq look simple. The battlefield there includes Russia and Turkey, major military powers with separate agendas.

Currently, the U.S. has only about 300 American troops on the ground in Syria, and no artillery or attack helicopters positioned near Raqqa. Even fixed-wing air support and intelligence aircraft may be in short supply until the Mosul operation is over, Jeffrey said. There may be urgency, he added, but "we’re not set up for Raqqa."

It will be difficult to keep the U.S. allies focused on ISIS militants rather than each other. The new president may have to consider authorizing more American boots on the ground for Syria to help manage the fractured network of allies.

"The biggest problem with Raqqa will be managing the coalition," said J. Matthew McInnis, a Middle East security expert with the American Enterprise Institute. "If you get an extra six hundred or an extra one thousand troops, that doesn’t dramatically change the situation from a military standpoint, but it does from a political standpoint. You gain a certain amount of ability to manage the situation when you have a little bit larger number troops there."

The invasion of Raqqa will put the teams of U.S. special operations troops into a unique role managing the movements of rival allied factions that often have fought each other during the five-year-old civil war.

Small teams of American troops will attach to various allied elements, which could include the Turkish military, Syrian Kurdish militias, Sunni Arab tribal fighters and others linked to the so-called Syrian Defense Force, according to defense officials familiar with the planning. Those teams of elite American troops will provide vital communication links between the groups as well as to the U.S.-led coalition’s centralized command and control system overseeing the operation.

"The U.S. can play a very pivotal role in negotiating this, and then the U.S. troops would help them deconflict it on the ground," said Andrew Tabler, a Syria expert with the Washington Institute for Near East Policy. "I don’t know who else can play that role."

The main fissure in the U.S. alliance is between Turkey and the Syrian Kurds.

The U.S. military believes that the only rebel faction capable of fighting ISIS in Raqqa is the Syrian Democratic Forces, or SDF, a network of militias dominated by the Syrian Kurds. Syrian Kurdish fighters have proven to be an effective ally for fighting ISIS with the help of U.S. air strikes in other parts of Syria.

This frame grab from video provided by Arab 24 network shows officials with the U.S.-backed Syria Democratic Forces at a press conference in Ein Issa in northern Syria. U.S.-backed Kurdish-led Syrian forces announced their plan Sunday, Nov. 6, 2016, to retake the Islamic State group's de facto capital of Raqqa, saying they hoped Turkey would not "interfere in internal Syrian affairs."

Photo Credit: Arab 24 network via AP

Yet Turkey — a NATO ally — fiercely opposes U.S. support for the Kurds, fearing that a strengthened Kurdish force will consolidate power and form its own autonomous region along Turkey’s southern border. For decades Turkey has mounted counter-insurgency operations targeting its own restive Kurdish minority.

This tension erupted in combat recently. On Oct. 20, the two sides exchanged indirect fire, prompting the Turkish military to launch air strikes against Kurdish forces. Turkey said its attacks killed 160 to 200 Kurdish fighters.

The U.S. also depends on Turkey for use of Incirlik Air Base, a hub for U.S. air operations near Turkey's southern border. A power struggle has played out in public view as the top American general overseeing operations against ISIS vowed to march on Raqqa with Kurdish forces regardless of Turkey's opposition. "Turkey doesn’t want to see us operating with the SDF anywhere, particularly in Raqqa," Lt. Gen. Stephen Townsend acknowledged in a recent press briefing.

After Sunday's announcement, Marine Corps Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, met with his Turkish counterpart, Gen. Hulusi Akar, to discuss next steps, according to a Pentagon spokesman. Details of their meeting in Turkey were not disclosed, except that they agreed to develop "long-term plans to work collaboratively ... and consult closely on the coalition plan to seize and hold Raqqa," said Navy Capt. Greg Hicks.

There is a sense that the U.S. and Turkey may resolve their differences in a way that makes the battlefield even more complicated and result in more American personnel on the ground, said McInnis. "The best case scenario for the U.S.," he added, "is to move toward some zones of operation, that we try to stay out of each other’s way. Having additional U.S. troops in Syria would help manage the balance of power and manage the situation with Turkey."


THREAT SPREADS BEYOND THE CALIPHATE

America's next president will face the added challenge of confronting the ISIS threat beyond Iraq and Syria. In Libya, for instance, ISIS loyalists are clinging to a small outpost in the port city of Sirte despite months of U.S. airstrikes.

In Afghanistan, U.S. forces are supporting operations against an estimated 1,000 militants who've declared allegiance to the Islamic State. They're located primarily in Nangarhar province, along the Pakistan border. The top American commander there, Army Gen. John Nicholson, told NBC Newslast month that those fighters are attempting to establish a caliphate.

A small cadre of U.S. troops in Egypt’s Sinai Peninsula live under constant threat from local militants loyal to the Islamic State group.

And the movement is slowly spreading east.

"It’s clear to me that [ISIS] is also ‘rebalancing’ to the Indo-Asia-Pacific," Adm. Harry Harris, the head of U.S. Pacific Command, said in June. There are signs of ISIS and ISIS-linked groups in Bangladesh, Indonesia, Malaysia and the Philippines.

The group has the potential to expand across many parts of Asia, where governance is weak, Karasik said. "The geographical dispersion is heading toward south and southeast Asia, and also potentially up into central Asia and southern Russia and ultimately to China," he added. "The group seems to be looking east to find new ungovernable urban areas that they can take root in."

The group spreads in two ways. In some cases, the ISIS leadership strategically identifies a location that could be a safe haven, as they did in Libya. Last year, thousands of ISIS fighters migrated from Iraq and Syria to the ISIS enclave in Libya.

"That," Ucko said, "was a rational strategic evolution by the group." At the same time, ISIS spreads its influence with grassroots support, when local militant groups pledge allegiance and seek to align with ISIS leaders.

"Nothing gains the world’s attention like a black flag with Arabic writing on it."

Tilghman is Military Times' Pentagon bureau chief. Follow him on Twitter: @andrewtilghman. Military Times chief editor Andrew deGrandpre contributed to this report. Follow him on Twitter: @adegrandpre.