MOSUL, Iraq (AP) — Iraq's prime minister declared victory over the Islamic State group in Mosul after more than eight months of some of the toughest fighting Iraqi forces have faced in the more than 3-year-old war against the extremists.

Iraqi and coalition forces acknowledged from the start that Mosul would be a challenge, but the Iraqi leadership's initial vows that it would be over by the end of 2016 underestimated the capacity and the resolve of the IS fighters left to fight to the death.

Here are five things to know now that the fight for Mosul is officially over.

THE IMPORTANCE OF MOSUL

Mosul held deep symbolic importance for IS. It was after the Islamic State group overran Mosul in June of 2014 that they declared a caliphate stretching from territory in northern Syria deep into Iraq's north and west. And it was from Mosul's al-Nuri mosque that the group's leader, Abu Bakr al-Baghdadi, made his only public appearance when he gave a Friday sermon calling on all Muslims to follow him as "caliph." He vowed that IS would conquer "Rome," and the entire world.

Mosul was also the bureaucratic and financial backbone of IS. In the early days of IS growth in Iraq and Syria, former Saddam-era military officers from Mosul made up some of the group's highest-ranking members and helped garner supporters in Iraq.

Raiding Mosul's central bank, and taxing and extorting the city's wealthy inhabitants, made IS the richest terrorist organization in the world in the summer of 2014. Mosul's vast industrial zones were converted into factories for weapons and explosives.

HOW U.S.-BACKED IRAQI FORCES WON

Iraq spent more than two years rebuilding its armed forces and preparing for the Mosul offensive. Some 70,000 forces drawn from Iraq's army, special forces, the federal police, and tribal and militia fighters were mobilized for the fight, according to Iraq's joint operations command.

Initially, Iraqi forces planned to attack the city simultaneously from multiple fronts — north, east and south. But forces without urban combat experience and limited training quickly proved incapable of leading pushes and instead were moved to act largely as holding forces. Iraq's special forces and the federal police led most combat operations to retake Mosul, first clearing the city's east and then the west.

U.S.-led coalition support for Iraqi ground forces in Mosul repeatedly proved to be the critical factor. Iraqi forces repeatedly called in airstrikes to kill just one or two IS fighters armed with light weapons. Iraqi commanders said this approach was adopted to keep military casualties to a minimum.

The final battles for Mosul played out in the Old City, dense terrain measuring just a few square kilometers (miles) where the United Nations estimated IS held more than 100,000 people as human shields.

THE COST OF VICTORY

The extent of the cost in civilian life is not yet known as many bodies are still believed to be trapped under rubble and Iraq lacks a centralized system for documenting civilian casualties.

But at a minimum hundreds of people are believed to have been killed and thousands more wounded, according to interviews conducted with doctors at field clinics and humanitarian organizations.

Iraq's military has also suffered high casualty rates. The government does not disclose official death tolls, but many of the units leading assaults in Mosul faced attrition rates of upward of 25 percent when engaged in urban combat. The high casualty rates will likely undermine the forces' ability to continue the fight against IS in the pockets of territory they still hold in Iraq.

DESTRUCTION

The levels of destruction are dramatically different between Mosul's east and west. Complexes in the east used by IS fighters, like the city's university, were heavily bombed. But many of the east's residential neighborhoods suffered relatively little damage.

In the west, however, entire city blocks are damaged or destroyed by months of airstrikes and artillery. It was also in western Mosul that a single U.S. airstrike killed more than a hundred civilians sheltering in basement of a single home.

Human rights and aid groups warn that such widespread destruction could undermine the military victory and make it more difficult for the hundreds of thousands of civilians who fled the west to return to their homes.

THE FIGHT CONTINUES

The operation in Syria to retake the IS group's de-facto capital, Raqqa, is in full swing, with U.S.-backed, Kurdish-led forces having retaken a handful of neighborhoods. But the battle for Raqqa is expected to be extended and bloody, and IS still controls a number of other towns in eastern Syria.

IS also continues to carry out insurgent attacks in Iraq, Syria and beyond. Iraqi and coalition officials have said the fight against IS as an insurgent force will likely be more difficult than the fight against the group as a conventional one and that the extremists will continue to pose a threat for years to come.