ANALYSIS: ISIS 'caliphate' facing collapse, and fallout could reach the West As Mosul falls, question remain about the future of the region.

 -- As Mosul continues to fall to members of Iraq’s U.S.-backed special forces, ISIS’ so-called caliphate faces collapse. For the people of Asia, Europe and the United States, however, the terrorist organization is likely to remain a threat.

ISIS controls less than a square mile of land in Mosul right now, and displaced people are flowing out of that zone on foot, sometimes carrying severely wounded civilians with them.

Roughly 300 Islamic State fighters remain in Mosul’s Old City, according to a report by The Associated Press.

Concerns are rising that the rush to fill the vacuum left behind by the terrorist group could lead to an even greater conflict across the region.

Furthermore, ISIS’ tactic of pursuing smaller attacks in Western cities — like those that have recently unnerved the U.K. — is likely to continue and could increase in intensity if the group’s foreign fighters in Syria and Iraq return home to the West.

A fight to the death in Mosul

The last days of ISIS control of Mosul have arrived.

A pocket of their fighters exists in the Old City next to the Tigris River, which divides east and west.

People have different predictions about when the battle will be over, but some expect that a formal announcement will be made in the coming days.

ISIS holdouts in western Iraq and Syria, as well as a number of other countries, may continue to attack and bomb after any declaration of victory on the part of special forces in Mosul.

For now, there’s a humanitarian crisis that has affected survivors.

Thousands of people have fled their homes during the fighting. Many of them have no houses to return to, after the relentless aerial bombing there.

Mosul residents are traumatized, and the scene bears similarities to what happened to Dresden at the end of World War II, when allied forces ousted the Nazis from Germany.

A long battle ahead in Syria

The U.S.-trained, -equipped and -backed Syrian Democratic Forces (SDF) are moving forward and have entered Raqqa, the capital of the “caliphate,” from a number of sides. The group, known as the People’s Protection Units (YPG), is a mix of Arabs and Kurds.

The YPG has been fighting ISIS for years and is now considered a battle-hardened opponent.

Based solely on Raqqa’s physical makeup, defeating ISIS there will likely be easier than in Mosul. There is far less terrain in the city, and fewer residents are holed up inside it.

Fighting in the Euphrates River Valley

ISIS controls a swath of territory along the Euphrates River Valley, which snakes southeast from Raqqa to the Syria-Iraq border.

Questions persist about what groups will take the fight to ISIS in this region: The Pentagon seems reluctant to send U.S. troops, effectively ceding territory to Syrian President Bashar al-Assad.

Key flashpoints in this region include Deir ez-Zour, Mayadin and Tanf.

Who fills the vacuum left by ISIS?

Since the end of World War II, there have been few conflicts in which so many national and international players were vying for territory and interests.

In the desert from eastern Syria to western Iraq, a diverse collection of parties is converging.

In Syria there is Assad’s regime, along with Iranian, Russian and Lebanese (Hezbollah) fighters. The SDF and Kurdish and Syrian rebels form another side of this conflict and are backed by Western allies, including the United States.

Islamist Syrian rebels also have a presence. This group includes former al-Qaeda fighters and Turkmen, backed by Turkey and Qatar and supported by sympathetic Sunni Muslims across the region.

In Iraq, there are potential conflicts between Kurdish groups and the Iraqi government and between Kurdish and Shiite militias.

The situation is messy, meaning that long after Mosul falls, the territory now claimed by ISIS will likely long be in dispute.

ABC News’ Michael Edison Hayden contributed to this report.