BAGHDAD • The battle for Mosul is expected to be difficult, involving street-to-street fighting with the 3,000 to 4,500 Islamic State in Iraq and Syria (ISIS) militants estimated to be in the area.
ISIS is faced with an array of forces - the United States-led coalition, Iraqi soldiers, police and mainly Shi'ite Muslim Hashed al-Shaabi forces, as well as Kurdish Peshmerga fighters.
This coalition of myriad and sometimes rival Iraqi forces will have to fight through ISIS defences - in some cases, over distances of dozens of kilometres from their current positions - to reach the city.
The operation will be supported by the US-led coalition, while Iranian forces will work with some Shi'ite militia groups.
Turkey also has troops at a base near Mosul that it has declined to remove despite Baghdad's demands for it to do so.
The role of the Hashed al-Shaabi militia forces is contentious, and Sunni politicians oppose their entry into Sunni Arab Mosul.
Hashed forces may ultimately be diverted to retake the town of Tal Afar, west of Mosul, giving them a role in the operation while keeping them out of the city. There is also the contentious issue of post-Mosul territorial control, with Iraqi Kurdistan wanting to maintain control of areas that are claimed by both it and the federal government in Baghdad.
The Mosul offensive could affect more than one million people, according to the United Nations. These displaced people would come on top of some 3.38 million Iraqis who have already left their homes since January 2014.
The UN refugee agency has appealed for US$584 million (S$811 million) to help those displaced, and has so far received only a part of that amount.
Washington last month announced aid of more than US$181 million.