Raqqa, the Islamic State’s de facto capital, fell last week. It was a significant milestone, but more like the end of the beginning than the beginning of the end in the fight against the terrorist group.

Once the self-proclaimed “caliphate” loses the rest of its rapidly diminishing territory, the Islamic State, also known as ISIS, will return to its underground roots and plot a comeback. This is precisely what its forerunner, the Islamic State of Iraq (formerly Al Qaeda in Iraq), did after being decimated by the Bush administration’s surge of American troops.

When U.S. military forces withdrew from Iraq in 2011, U.S. intelligence officials estimated that the Islamic State of Iraq had been whittled down to about seven hundred fighters. Although defeated on the battlefield, the group bided its time. Patience paid off. When disenfranchised Sunni Arabs in Iraq and Syria rebelled against what they regarded as hostile and corrupt non-Sunni governments in Baghdad and Damascus, the Islamic State of Iraq was ready to exploit the situation.

The successful revolt bred more success. By 2013, the group was boosted by an influx of Syrian and foreign volunteers, who mobilized against Syrian president Bashar al-Assad’s brutal regime. The organization renamed itself the Islamic State of Iraq and Syria.

By late 2014, ISIS had seized much of eastern Syria and western Iraq, and declared the formation of a “caliphate.” It attracted more than forty thousand foreign fighters and ruled over roughly ten million people in territory the size of Maryland.

Iran is now trying to twist the Syrian kaleidoscope, repositioning itself and its surrogates to exert greater pressure on Israel. An Iranian-controlled Iraqi Shiite militia deployed in Syria, Hezbollah al-Nujaba, has formed a “Golan Liberation” force and publicly declared itself ready to take action against Israel. Both Iran and Israel appear to be preparing for a war in Syria and Lebanon.

Turkey, which initially intervened in Syria in support of rebels fighting against the Assad regime, now appears to be focused more on securing its border, defeating the Kurdish insurgency in eastern Turkey, and blocking Syrian Kurds from establishing an independent state that could become a source of support for Kurdish separatists inside Turkey.

The coming shift in U.S. policy

The future of U.S. policy remains the big question mark in Syria. President Donald Trump said that, after the fall of Raqqa, the United States “will soon transition into a new phase” in Syria and offer support to local security forces.

But the scope, scale, and duration of such U.S. support remains unclear, particularly in light of the president’s known aversion to entangling alliances and nation-building.

The United States has deployed more than five hundred special operations forces in Syria and about five thousand military personnel in Iraq, primarily in training and advisory roles.

Pentagon officials are preparing for the next phase of the campaign against the Islamic State. They will support an SDF offensive to roll up Islamic State forces by advancing down the Euphrates River toward the Iraqi border, which is where they are expected to link up with Iraqi forces advancing on the other side of the border.

But these plans may have been disrupted by the swift advance of Iranian-led Syrian and Hezbollah forces. Supported by Russian air strikes, those troops have cut off the SDF’s most direct line of advance toward al-Bukamal, a key Islamic State stronghold near the border.

One particularly significant development was the construction of a Russian pontoon bridge, which enabled Iranian-backed forces to cross the Euphrates and prevented SDF fighters from advancing down the road on the east bank of the river. The river crossing signaled Russian support for Iran’s efforts to carve out a secure land bridge from Tehran to Damascus. It also violated the deconfliction agreement between Washington and Moscow.

The Trump administration must decide how to push back against Russia’s increasingly confrontational behavior in Syria. Moreover, it must decide how best to contain and roll back Iran’s growing influence in Syria and elsewhere.

The Kurdish-dominated SDF is one of the strongest military forces in eastern Syria. But it also presents political problems for Washington. The predominantly Arab population of Raqqa distrusts the Kurds and fears that Kurdish forces will impose a new occupation, rather than a liberation from the Islamic State.

U.S. support for Syrian Kurdish forces also is resented by Turkey, a NATO ally that is fighting its own war against the Kurdistan Workers’ Party (PKK), which has been designated as a terrorist organization by the United States and the European Union. Syrian Kurds made matters worse by raising a giant banner in Raqqa of PKK leader Abdullah Ocalan, who is jailed in Turkey on terrorism charges.

How deep and enduring is the Trump administration’s commitment to the Kurdish-led SDF? This is unclear. The alliance is the most viable current option for blocking Iranian/Syrian consolidation of control over eastern Syria, but it also could undermine U.S. relations with Turkey and with Sunni Arabs who soon will chafe over Kurdish rule.

The bottom line is that, with or without the SDF, Washington must remain engaged in Syria and Iraq after the military defeat of the Islamic State. It is the only way to prevent an ISIS re-emergence and to offset Iran’s efforts to twist the regional kaleidoscope into a pattern more favorable to its own interests.