When George W Bush sent the US into Iraq in 2003, he believed he would be replacing Saddam Hussein with a peaceful, pro-American Arab democracy that would naturally look to the Christian west for support. In reality, seven years on, it appears that he has instead created a highly radicalised pro-Iranian sectarian killing field, where most of the Iraqi Christian minority has been forced to flee abroad.

This week saw new levels of violence directed at Iraq's Christians. Eight days after the attack on Baghdad's main Catholic church that left more than 50 worshippers dead, militants detonated more than 14 bombs in Christian suburbs, killing at least four and wounding about 30. Since then the Islamic State of Iraq (ISI), an al-Qaida front, has warned of a new wave of attacks on Christians "wherever they can be reached … We will open upon them the doors of destruction and rivers of blood."

Before Bush senior took on Saddam for the first time in 1991, there were more than a million Christians in Iraq. They made up just under 10% of the population, and were a prosperous and prominent minority, something exemplified by the high profile of Tariq Aziz, Saddam's Christian foreign minister. Educated and middle class, the Christians were concentrated in Mosul, Basra and especially Baghdad, which then had the largest Christian population of any city in the Middle East.

Of the 800,000 Christians still in Iraq when Dubya unleashed the US army on Saddam for the second time, two thirds have fled the country. In 2006, a priest was kidnapped, then found beheaded and dismembered; 15 churches have been bombed and many other priests killed. Iraqi refugees tell me that Christian women have suffered kidnap and rape, little of which has been reported.

The Christian community in Iraq is one of the oldest in the world: according to tradition it was St Thomas and his cousin Addai who first brought Christianity to Mesopotamia, soon after the crucifixion. At the council of Nicea, where the words of the creed were thrashed out in 325AD, there were more bishops from Mesopotamia than from western Europe. Later, the region became a refuge for groups considered heretical by the Orthodox Byzantines – such as the Mandeans, the last surviving Gnostic sect in the world, who follow what they believe to be the teachings of John the Baptist; and the Church of the East, or Nestorians, who played an important part in bringing Greek philosophy, science, mathematics, astronomy and medicine first to the Islamic world and then the universities of medieval Europe.

But with the collapse of the Ottoman empire, religious minorities fled to places where they could be majorities, while those too few for that have abandoned the region altogether, seeking out places less heavy with history, such as the US or Australia. Today, the Christians are a minority of 10 million in the Middle East, struggling to keep afloat amid 190 million non-Christians. In the last 20 years at least four million have left to make new lives for themselves in the west.

This haemorrhage accelerated after the ill-judged post-9/11 Anglo-American adventures in the Islamic world, and particularly after Bush used the word crusade, which in the eyes of many Muslims implicated the Arab Christians in a wider crusader assault on the Muslim world. So it was that two invasions that were intended to suppress terrorism actually had the reverse effect, radicalising the entire region.

According to the historian Professor Kamal Salibi, of the American University of Beirut, the Christians have simply had enough: "There is a feeling of fin de race among Christians all over the Middle East," he told me. "It's a feeling that 14 centuries of having all the time to be smart, to be ahead of the others, is long enough. The Arab Christians tend to be well qualified, highly educated. Now they just want to go somewhere else."

Certainly, for the first time, that now looks like being a possibility in Iraq: last week Michael Youash, of the Iraq Sustainable Democracy Project, warned that in the near future "perhaps we'll find no Christians in Iraq". Given the overt Christian faith of the two architects of the invasion, Bush and Tony Blair, there is a tragic irony in the fact that their most lasting contribution to the region may well be to have created the environment that led to the destruction of Christianity in one of its ancient heartlands – something Arab, Mongol and Ottoman conquests all failed to pull off.