The drones are especially dangerous. Islamic State fighters have been sending more and more unmanned aircraft into the air, according to the Iraqi army. Buzzing overhead, the ground-controlled drones release deadly explosives down upon the ranks of their opponents.

In the mean time, the Iraqi military has adapted to the aerial threat, but the drones - small and, therefore, difficult to distinguish from afar - are still dangerous, particularly in street battles, when soldiers don't have the time to keep an eye on what might fly overhead.

The battle for Mosul is "the toughest and most brutal phase of this war," an American officer told online defense magazine Breaking Defense.

It's "probably the toughest and most brutal close quarters combat that I have experienced in my 34-year career," he said.

Read: Mosul's displaced

Battle against suicide bombers

Reports from the Iraqi and American military have also indicated this, pointing to the many times IS suicide bombers have tried using explosive-laden vehicles to hit deep into the ranks of their opponents.

According to Breaking Defense, the Iraqi military has prepared itself better for this danger. In order to stop fast-approaching vehicles, each troop has soldiers armed with grenades.

As soon as they retake a street, soldiers erect soil embankments to leave no chance to suicide bombers. But this certainly does not protect them from the many explosive traps IS has left, for example, in field hospitals that look abandoned.

These difficulties pose not only strategic, but also ethical challenges. Moral catastrophes are mapped out. On March 17, for example, the US military shot a vehicle carrying explosives. The explosion razed a house where IS had assembled civilians to act as human shields. Some 200 people were killed in the blast.

Read: No end to 'IS' murders of Yazidis

Reactions among the civilian population

Anti-IS forces have retaken approximately three-quarters of Mosul. Nevertheless, they are preparing for a longer battle. For one thing, IS fighters have entrenched themselves in the Old City, which is difficult to enter and which they've fortified over the past two years. The mission to eradicate these fighters will mean taking the quarter house by house.

Roughly half a million civilians have fled the city amid intense fighting. A small pocket of the civilian population supports IS. They justify their support of the terrorist group by pointing to the Iraqi government's policies, which - in their opinion - favor the minority Shiites in power.

Others joined IS following massive death tolls inflicted by the Iraqi army and its allies. Breaking Defense quoted a Sunni clan leader, who estimated that hundreds, if not thousands, had been killed in Mosul. Moreover, Shiite militias have seized numerous Sunni villages around Mosul.

"We fear that the use of excessive force will cost the lives of thousands more civilians, creating hardships and hard feelings that will only set the stage for the next IS, or worse," he said.

Read: Ex-Iraqi soldier on trial in Berlin for war crimes

Political process indispensable

Between 8,500 and 12,630 civilians in both Iraq and Syria have been wounded since the beginning of the anti-IS mission began in August 2014, according to journalist-led transparency project Airwars. The figures extend through April 18 of this year. Airwars estimates that between 3,100 and 5,000 civilians were killed in assaults carried out during that time period.

In light of these figures, the US military says a political process must flank the military's battle in order to prevent Sunnis from both countries allying themselves with Islamic State.

Read: Refugees in Germany lend a hand to displaced Iraqis

A future in Iraq

A military victory over IS can only be a beginning, says former US General David Petraeus, who battled against the Sunni uprising in Iraq from 2007 to 2008. The insurrection saw an alliance between the local population and al Qaeda.

"The military defeat of ISIS is only the first step. The much more challenging task is to use all elements of American and coalition power to help achieve political solutions that will avoid once again creating fertile ground for extremists, and thereby avoid the rise of IS 3.0," Petraeus told Breaking Defense in an email.

According to US commander Stephen Townsend, this means offering prospects to the oppressed Sunni population.  They need to be convinced they have a future in Iraq. If this works, says Townsend, a long-term peace in Iraq would be possible. If it fails, terror and war will continue.