Iraq’s prime minister announced recently that it is fighting to take back Mosul from Islamic State militants. Mosul, Iraq’s second-largest city, has been occupied by IS fighters for more than two years.

When IS captured Mosul, it was a diverse city of more than 2 million people. These included, according to the BBC, Sunni Arabs, Kurds, Assyrians and Turkmens. The city is important to IS because it is where IS leader, Abu Bakr al-Baghdadi, declared that he and his terrorist organization were forming a caliphate in parts of Iraq and nearby Syria.

According to USA Today, there are as many as 4,500 IS fighters in Mosul. The BBC estimates that the number may be as high as 8,000. Taking back Mosul won’t be a cakewalk, according to USA Today, as IS forces have likely fortified the city and built up defenses during their occupation.

Because success is not guaranteed in the battle, which analysts predict will be slow and lengthy, Iraqi forces went in with guns blazing. USA Today reports that more than 30,000 U.S.-trained troops — 12 brigades — are battling against IS forces to get them out of the city.

The challenge for Iraqi forces is fighting in an urban environment, as military strategists have no doubt that IS will use the more than 1 million citizens who remain in the city as human shields.

According to Deutsche Welle, one of Iraq’s top generals, Major General Najm al-Jabouri, is optimistic about retaking the city after the first day of fighting. An army spokesperson said, "We recorded a significant amount of recaptured territory in the first hours of the offensive, as well as weak resistance from IS," according to Deutsche Welle.

According to USA Today, the capture of Mosul will mean the end of the IS caliphate in Iraq. The recapture won’t, however, mean the end of IS or the radical Islamic ideology that undergirds Islamic State.

Deutsche Welle reports that the head of the Intelligence and Terrorism Information Center at the Center for Special Studies in Israel, Reuven Erlich, expects the expulsion of IS from Mosul to lead to more politically and socially savvy splinter groups.