Iraq on Monday celebrated the anniversary of its costly victory over the Islamic State group, which has lost virtually all the territory it once held but still carries out sporadic attacks.

The government declared victory last December after a grueling three-year war in which tens of thousands of people were killed and hundreds of thousands displaced. Entire towns and neighborhoods were reduced to rubble in the fighting.

The government declared Monday a national holiday, and a moment of silence is planned for later in the day. Checkpoints in the capital were decorated with Iraqi flags and balloons, as security forces patrolled the streets playing patriotic music.

As part of the celebrations, authorities plan to reopen parts of Baghdad's fortified Green Zone — home to key government offices and embassies — to the public. The move is billed as an act of transparency following protests against corruption and poor public services.

The Islamic State group, which traces its roots back to the insurgency that followed the 2003 U.S.-led invasion, swept into Iraq from neighboring Syria in the summer of 2014. It carved out a self-styled caliphate across a third of both countries, imposing a harsh form of Islamic rule and massacring its opponents. The group abducted thousands of women and girls from the Yazidi religious minority and forced them into sexual slavery.

Iraqi troops, along with Iranian-backed paramilitary groups and a U.S.-led coalition, eventually drove the group from all the territory it once held in Iraq, including in the climactic battle for Mosul, the country's second-largest city. IS still holds a small pocket of territory in Syria, near the Iraqi border.

Iraq is still grappling with the legacy of the extremist group's brutal rule.

More than 1.8 million Iraqis remain displaced across the country, and a staggering 8 million require some form of humanitarian aid, according to the Norwegian Refugee Council. Those with suspected links to IS have been rejected by their communities, while thousands of children fathered by IS militants — including those born to enslaved Yazidi women — are still unrecognized by the state.

Nearly two-thirds of displaced people say they are unwilling or unable to return home in the next year, with more than half saying their homes were damaged or destroyed, said the aid group.

"If this is what 'victory' looks like, then there is little to celebrate for millions of Iraqis still haunted by the crimes of the IS and the long war to eliminate it," said Norwegian Refugee Council Secretary General Jan Egeland. "They have largely been forgotten by their own government and the international community."