Iraqi Prime Minister Hayder Abadi has announced the beginning of the nation’s invasion of the city of Mosul today, the largest city held by ISIS, which ISIS forces have held for over two years. The invasion had been expected for some time, and Abadi had indicated the announcement was imminent just two days prior.

Iraqi planes have been dropping leaflets on the city over the past few days as well, advising locals the invasion was to begin soon, and telling them to remain in doors. The early offensive appears to be centered around US shelling of the city.

ISIS has been preparing for the invasion for months, digging moats full of oil, building walls to limit points of entry into the city, and building tunnels that will allow them to move quickly and covertly around the city. They’ve also set tire fires tonight, aiming to reduce visibility for warplanes.

While the Iraqi military has had relative success in previous offensives against ISIS cities in Ramadi and Fallujah, the offensive in Mosul is expected to be dramatically more difficult, not just because it is a larger city, but also because it is so much further away from the Iraqi military’s supply lines.

To make up for this further distance, the US is expected to play a much larger logistics role. The Iraqi military is also expected to more heavily rely on Kurdish Peshmerga, along with Sunni and Shi’ite militias in the fighting on the ground.

These diverse groups could quickly create problems in the invasion, as the Shi’ite militias were hugely problematic in previous invasions of Sunni cities, and the Peshmerga has been annexing most of the territory its gained recently, leading Abadi to warn that the Mosul border cannot be changed by this battle.

Also looming over the invasion is the question of Turkish military forces nearby, as the Turkish government has insisted it will play a role in the invasion, but has been warned against involvement by the Iraqi military, who says they aren’t welcome in Iraq to begin with.