BAGHDAD: One night last week, Islamic State militants in an SUV with tinted windows pulled up at the home of a former Iraqi army officer, one of the men they see as an obstacle to their goal of establishing a caliphate from Iraq to the Mediterranean.

As the retired major-general was led away to the vehicle draped in the trademark black and white flag, his son and wife feared the worst.

“I have been asking the families of other officers and no one knows why they were taken,” his son said by phone, breaking down in tears.

In the past week, militants who overran the city of Mosul last month have rounded up between 25 and 60 senior ex-military officers and members of former dictator Saddam Hussein’s banned Baath party, residents and relatives say.

The crackdown potentially signals a rift in the Sunni alliance that helped secure Islamic State fighters swift victory when they rode in from the desert to capture Mosul.

The northern city of around 2 million people is by far the largest to fall to the group now known as the Islamic State and a central part of its plans for a caliphate.

Tribes and former loyalists of Saddam’s Baath party were eager to hit back at Iraq’s Shiite leaders, even if they did not share ISIL’s vision of a caliphate ruled on Islamic precepts. But now, leaders of those groups are being ordered to swear allegiance to the new caliphate. “I think (the Islamic State) wants to give the message that they are the only group in the land, that people must follow them or give up their weapons,” said provincial governor Atheel Nujaifi, who is in touch with residents by phone after having fled to the Kurdish-controlled city of Arbil as Mosul fell.

Shiite parliamentarian Haidar Abadi said the Islamic State was taking pre-emptive action to head off potential challenges. “ISIL knows very well they can’t stay if these groups move against them. They are not giving them the opportunity.”

“ISIL called on their friends who are ex-Baathists to cooperate and they did. And now ISIL is kicking them out. Some will pledge allegiance. Those they don’t believe will pledge allegiance, they will execute,” he said.

An Iraqi national intelligence officer, confirming the arrest by militants of Saddam-era officers, said the motive was: “to panic people, or as revenge, or in the event that they would cooperate with the Iraqi government.”

Nujaifi, the governor, estimated that around 2,000 Mosul residents had signed up to join the Islamic State as fighters since they took the city. But he said career army officers and diehard Baathists were unlikely to be won over to ISIL.

Among those Nujaifi said had been rounded up by the ISIL were General Waad Hannoush, a Special Forces commander under Saddam, and Saifeddin Al-Mashhadani, a Baath Party leader featured as the three of clubs in the US Army’s “Iraqi Most Wanted” playing card deck during the 2003 US-led invasion.

The governor and some residents told Reuters that they believe ISIL’s bold declaration of a caliphate last week had caused local discontent, possibly prompting the group to act to head off the first stirrings of resistance.

The move echoes Islamic State tactics in neighboring Syria, where the group, an offshoot of Al-Qaeda, entrenched itself in the rebel-held east by eliminating other opponents of President Bashar Assad.