BAGHDAD - A year after the Islamic State group launched a brutally effective offensive, Iraq is struggling to survive as a unified nation, gripped by seemingly endless violence, sectarianism and humanitarian tragedy.

IS began the offensive on June 9, 2014, and overran a third of the country, declaring it and areas in neighbouring Syria a "caliphate" and carrying out atrocities from beheadings and mass executions to enslavement and rape.

The jihadists have been driven out of some areas, but still hold much of western Iraq and remain able to defeat Baghdad's forces and gain new territory despite a year of heavy fighting and some 4,000 strikes carried out in a 10-month US-led air campaign.

The Syrian civil war served as an incubator and training ground for IS, while widespread anger among Iraqi Sunni Arabs, who accused the Shiite-led government of marginalising and targeting their minority community, helped the jihadists succeed.

"The underlying causes of (the) IS rise are still there," said Patrick Skinner, an analyst with the Soufan Group intelligence consultancy.

"And that means IS will remain, perhaps kicked out periodically from place to place but still in the national bloodstream like a septic infection," he said.

IS overran Iraq's second city Mosul in less than 24 hours last year, despite being heavily outnumbered, then pushed south with allied militant groups, raising fears that Baghdad itself could fall.

The jihadists swept aside multiple Iraqi divisions, seized thousands of armoured vehicles, weapons and other equipment in a disaster that exposed the full scale of the incompetence and corruption within the security forces.

IS is known for the horrific abuses it has carried out -- including highly-choreographed beheadings recorded on video -- as much as its territorial gains.

In northern Iraq, the jihadists targeted members of the Yazidi faith in a campaign of kidnappings, enslavement and rape that the UN denounced as an "attempt to commit genocide".

And IS massacred hundreds of mostly Shiite military recruits along the Tigris River in Tikrit, killings that ultimately rallied support for Baghdad.

Tens of thousands of people volunteered to fight IS in response to a call from Grand Ayatollah Ali al-Sistani, Iraq's top Shiite cleric.

But Iran-backed Shiite militias -- some members of which took part in brutal sectarian violence in past years -- remain the core of what are known as the "popular mobilisation" forces.

They have been essential to gains Baghdad has made against IS, playing a major role in retaking one province and large parts of another, and are loved by many Iraqi Shiites.

But the power of the militias is also a threat to the Iraqi state, which claims to command them but does not control them, and they could also eventually turn on each other.

The borders, boundaries and demographics of Iraq have been drastically changed over the past year.

As IS advanced and security forces fled, Iraq's autonomous Kurdish region seized some disputed areas and consolidated its grip on others, helping it realise long-held territorial goals that Baghdad will find extremely difficult to reverse.

And widespread displacement -- nearly three million people since the beginning of 2014 -- has altered Iraq's demographic map, with Sunni Arabs fleeing to predominantly Shiite and Kurdish areas.

The conflict has drawn Washington back into a quagmire it thought it had escaped, making a mockery of its goal of Iraq being a stable, democratic ally in the region and leaving it with little to show for a hugely costly nearly nine-year war.

The US is carrying out air strikes against the jihadists and has sent thousands of soldiers to train Iraqi forces, and an international coalition has followed suit.

But the jihadists have proven highly resilient and last month even seized the city of Ramadi, the strategically important capital of Anbar province, which Iraqi forces had defended for over a year.

The fighting to drive IS back has wrecked town after town in Sunni-majority areas, leaving behind smashed houses, ruined shops and felled palm trees.

The damage will cause further resentment if it is not repaired, but doing so will add strain to an already cash-strapped government.

The destruction inherent in any war has been worsened by the burning and looting of homes and shops by some militiamen, which has contributed to mistrust of these forces.

And the war is destroying Iraq's past as well as its future, with IS smashing and bombing world-renowned historic sites, including Nimrud and Hatra, and looting priceless artefacts to fund its operations.