An unlikely array of forces is converging on the city of Mosul, lining up for a battle on the historic plains of northern Iraq that is likely to be decisive in the war against the Islamic State group.

The usual alliance - Iraqi troops alongside Shiite militiamen, Kurdish fighters, Sunni Arab tribesmen and US Special Forces - underscores the importance of this battle. Retaking Mosul, Iraq's second-largest city, would effectively break the back of the militant group, ending their self-declared "caliphate," at least in Iraq.

But victory doesn't mean an end to the conflict. In a post-Islamic State Iraq, the rivalries among the players in the anti-IS coalition could easily erupt.

The battle, expected near the end of the year, threatens to be long and grueling. If IS fighters dig in against an assault, they have hundreds of thousands of residents in the city as potential human shields. And as residents flee, they fuel the humanitarian crisis in Iraq's Kurdish region around Mosul, where camps are already overcrowded with more than 1.6 million people displaced over the past two years. Humanitarian groups are rushing to prepare for potentially 1 million more who could be displaced by a Mosul assault.

The biggest prize captured by the militants after they overran much of northern, western and central Iraq in the summer of 2014, Mosul has been vital for the Islamic State group. The reserves in its banks provided a massive cash boost to the group, and the city's infrastructure and resources helped IS as it set up its caliphate across Iraq and Syria.

Mosul was the location chosen by Islamic State leader Abu Bakr al-Baghdadi to make his first public appearance after declaring the caliphate, a triumphant sermon delivered at a historic mosque in the old city. For the past two years, much of the leadership seems to have operated from Mosul.

If Mosul is retaken, it would be a nearly complete reversal of the jihadis' 2014 sweep. The group would be left with only a few pockets of territory in Iraq. IS fighters have already responded to battlefield losses by reverting to guerrilla-style tactics or retreating into neighboring Syria to defend the group's territory there, which is also rapidly eroding.

For weeks, the disparate forces have clawed back territory in Nineveh province, where Mosul is located, seizing villages and key supply lines. Still, the Iraqi military's closest position is some 50 kilometers south of Mosul and there remain dozens of militant-held villages with civilian populations that the troops must take before reaching the city's outskirts. Kurdish forces are closer, some within 15 km of the city to the north and east.

US-led coalition forces have sped up training for Iraqi troops and Kurdish fighters, condensing courses that once took more than two months into just four weeks. In July, the Pentagon announced that 560 more US troops would deploy to Iraq to transform Qayara air base, south of Mosul, into a staging hub for the final assault.

Still, Iraq's military is thousands of soldiers short of the estimated 30,000 troops needed to launch the assault, and the existing forces are stretched thin trying to hold other recaptured territory, particularly in western Anbar province.

Iraq's "biggest challenge is generating the forces required to get to Mosul," said Major General Gary Volesky, the head of US ground forces in Iraq. "If you want to pull someone out of Anbar to go to Mosul, you've got to put somebody else there."