The extremist Islamist group ISIS, the Islamic State of Iraq and Syria, shocked the world this week by conquering the northern city of Mosul, one of Iraq's largest cities and home to some 2 million people.
ISIS is also a key player in the Syrian civil war, fighting both the regime of Syrian President Bashar Assad and more moderate rebel forces.
The group is made of Syrian, Iraqi and foreign fighters who slide back and forth between the two countries for attacks on opponents or to take safe haven in the territory they control. Mosul is by far their biggest conquest yet.
Here are several key things to know about the ISIS offensive:
1. The lightning advances haven't been as fast as they seem.
ISIS has been making gains throughout Iraq's western Anbar province and some of the areas around Mosul for months, but this received limited attention. ISIS seized the strategic, but smaller city of Fallujah in January. Fallujah is only about an hour's drive west of Baghdad. The group has also held areas farther to the west, near the Syrian border.
Throughout this period, Iraqi officials have pressed for more weaponry from the U.S., and have received some. Meanwhile, the U.S. and other nations have pressured Iraqi Prime Minister Nouri al-Maliki to reach out to these Sunni Muslim areas, which have been alienated by Maliki's Shiite-led government.
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