In One Map, The Dramatic Rise Of The Islamic State Militants

As this animated map shows, the extremist group now known as the Islamic State has made major gains since early last year with the declared goal of establishing an Islamic empire, or caliphate, in large parts of Iraq and Syria.

Capitalizing on the turmoil in those countries, the group's fighters have been able to move back and forth with relative ease across the thinly populated deserts of eastern Syria and western Iraq.

IS began fighting and capturing towns last year in eastern and then northern Syria, as the initial red dots, from January 2013, show on the map. Its battle with President Bashar Assad's government has expanded to include frequent clashes with the other rebel groups that are also fighting Assad's regime.

After the conquests in Syria, IS began moving aggressively into western Iraq and taking control of multiple towns. The group scored a major victory in January this year by taking the city of Fallujah, just an hour's drive west of Baghdad.

IS scored its biggest triumph in June when it overran the northern city of Mosul, one of Iraq's largest cities and home to some 2 million people. Iraqi troops folded without a major fight, and lS has followed up by pushing farther south and taking more cities on the road to Baghdad.

The group now controls many cities and towns in the Sunni areas west and north of Baghdad. In many areas, it works in alliance with supportive local militias and tribes. But it has encountered tougher resistance as it starts to reach Shiite areas closer to the capital.

It's worth noting that the Euphrates and Tigris rivers, which gave rise to the earliest civilizations, also feature prominently in the latest fighting. In this desert region, most towns and cites are built along the rivers.

Last year and early this year, IS captured population centers up and down the Euphrates River, on both sides of the Syria-Iraq border. After the capture of Mosul, which straddles the Tigris River, the group advanced into towns farther south along the river.

IS, which is led by an Iraqi, Abu Bakar al-Baghdadi, was initially linked to al-Qaida. But the al-Qaida leadership deemed its tactics so extreme that it effectively severed the ties. Since the rupture, IS has operated largely on its own, but that has not slowed the group down. Some analysts see IS as rivaling, if not surpassing, al-Qaida as the single most powerful jihadist group.

IS has alienated many civilians in Syria with its extreme interpretation of Islamic law. Yet in just over a year, IS has become a powerful force fueling the conflicts in both Syria and Iraq.