My city was supposed to be the model for a better tomorrow in Iraq. Integrated security forces from all ethnic groups, restored cohesion among the many segments of Iraqi society – this was the hope amid the surge, back when I was the mayor in Tal Afar.

But that was nearly a decade ago, and now my city is a battleground again, as government security forces attempt to withstand the march of Sunni militants, as the incubator for an Islamic state has turned into sectarian chaos. The dream of a unified Iraq has not just been deferred but destroyed.

Isis was a sleeping giant, and to see what went so wrong, you have to follow the destructive path set out by the United States as an occupying power in my country, almost from the moment those first air strikes began.

Back in 2003, most Shia Muslims and a good number of Kurds welcomed the Americans. The Sunni population, meanwhile, was not of one mind: many of them were outright opposed to US control, while others were holding out for things to change for the better. Give the occupiers six months, argued Sunni scholars, to see what happens. Iraq had suffered so many calamities – so many wars and siege after siege – that a population suffering in poverty and destitution, no matter one's ethnic background, seemed willing to hope together.

Then the American occupational authority, led by Paul Bremer, dissolved state institutions (including the Iraqi army), uprooted the Ba'ath party (by way of harsh de-Baathification laws) and, even worse, failed to give adequate Sunni representation in the "transitional" government (five Sunni Muslims sat on the original Governing Council, to 13 Shia representatives, five Kurds and one Turkoman). This was the beginning of the end of that better tomorrow for Sunni people in Iraq.

The calls for anti-American resistance began then and there, in the summer of 2003, opening a door that never shut – Sunni Muslims were stifled, so multiple groups vied for military power. It could have been a new golden era for getting along, but any measures that might have restored cohesion among the many segments of Iraqi society were quickly lost on the Americans – and chaos began to sow amongst the locals.

There was no longer the phenomenon of common hope – only the reality of sectarianism everywhere. Where there had been inter-sect marriages – loving tribalism, not angry sectarianism – now came threats to authority, and power grabs everywhere you looked.

Unfortunately, those who opposed the new US regime were entrenched behind ethnicity and sectarianism to the extent that they lost any chance for building a national identity, which was not only diminished but disappeared. Nobody called themselves Iraqi anymore. Everyone became a Kurd or an Arab or a Turkman, and so on and so on.

By 2006, with General George Casey about to lead the American surge, there was civil war. Then came Petraeus, with the squashing of al-Qaida in Iraq but the rise of a Sunni awakening in the Sons of Iraq movement, many members of which would later come to fight for Isis. Indeed, by the time US forces had departed in December of 2011, terrorist organizations had returned swiftly to the scene – day after day, year after year, and even in my town.

Kurds disputed oil lands, politicians fought more than ever, and the Shia prime minister Nuri al-Maliki, having been given a second term in 2010, reneged on his promises. I was no longer mayor in Tal Afar, but you could sense the crisis plaguing us everywhere. You could hear the Sunni demonstrators demanding their rights, and you could see the government dealing poorly with their manifestations of violence. By the time security forces broke up the protests in December of last year and arrested Ahmed al-Alwani, the Sunni MP who backed them, you could almost hear, in the faint distance of a land plagued by occupied history, the sound of Isis rumbling into town.

One arrest may not have been the straw that broke the camel's back – the militants have taken years of careful planning – but the straws have been piling on for years. Now I fear my town will collapse under the weight of them, when the rest of the world ignored a force that has been building since day one.

• This op-ed was translated from the Arabic by Joseph Halabi.