The US said the fall of Iraq's second city posed a threat to the entire region. It may also have made ISIS the most cash-rich militant group in the world.
Initially, the group relied on donations from wealthy individuals in Gulf Arab states, particularly Kuwait and Saudi Arabia, who supported its fight against President Bashar al-Assad.
Today, IS is said to earn significant amounts from the oil fields it controls in eastern Syria, reportedly selling some of the supply back to the Syrian government. It is also believed to have been selling looted antiquities from historical sites.
Prof Neumann believes that before the capture of Mosul in June 2014, IS had cash and assets worth about $900m (£500m). Afterwards, this rose to around $2bn (£1.18bn).
The group reportedly took hundreds of millions of dollars from Mosul's branch of Iraq's central bank. And its financial windfall looked set to continue if it maintains control of oil fields in northern Iraq.
Inter-rebel tension
The group has been operating independently of other jihadist groups in Syria such as the al-Nusra Front, the official al-Qaeda affiliate in the country, and has had a tense relationship with other rebels.
Baghdadi sought to merge with al-Nusra, which rejected the deal, and the two groups have operated separately since.
Zawahiri has urged IS to focus on Iraq and leave Syria to al-Nusra, but Baghdadi and his fighters openly defied the al-Qaeda chief.
Hostility to IS grew steadily in Syria as regularly attacked fellow rebels and abused civilian supporters of the Syrian opposition.
In January 2014, rebels from both Western-backed and Islamist groups launched an offensive against IS, seeking to drive its predominantly foreign fighters out of Syria.
Thousands of people are reported to have been killed in the infighting.