Former Saddam-era officials brought expertise that has propelled rise of Islamic State

While attending the Iraqi army's artillery school nearly 20 years ago, Ali Omran remembers one major well; an Islamic hardliner who chided Omran for wearing a flag pin bearing the words "God is great" into the bathroom - a "defiled place to bring the name of the Almighty".

Omran didn't see Major Taha Taher al-Ani again until years later, in 2003. The Americans had invaded Iraq and were storming toward Baghdad. Saddam Hussein's fall was imminent.

At a military base north of the capital, al-Ani was directing the loading of weapons, ammunition and ordnance into trucks to spirit away, military hardware he took with him when he joined Tawhid wa'l-Jihad, a forerunner of al-Qaeda's branch in Iraq.

Now al-Ani is a commander in Islamic State, said Omran, who rose to become a major general in the Iraqi army and now commands its 5th Division fighting IS. He kept track of his former comrade through Iraq's tribal networks and intelligence gathered by the government's main counterterrorism service. It's a common trajectory.

Under its leader, Iraqi jihadi Abu Bakr al-Baghdadi, IS's top command is dominated by former officers from Saddam's military and intelligence agencies, according to senior Iraqi officers.

Their experience is a major reason for the group's gains in large parts of Iraq and Syria.

The officers gave IS the organisation and discipline it needed to weld together jihadi fighters drawn from across the globe, integrating terror tactics like suicide bombings with military operations. They have been put in charge of intelligence-gathering, spying on Iraqi forces, maintaining and upgrading weapons and developing chemical weapons.