The Iraqi Army, which was expected to defend Mosul, fled the city as the ISIS attack began. ISIS had previously overrun the city of Fallujah. This week's advances give ISIS control over a vast swath of territory from the edge of Aleppo in western Syria to Mosul in Iraq and as far south as Fallujah.
In addition, ISIS forces have currently surrounded the Syrian border town of Deir el-Zour, one of the last pockets of resistance to ISIS in northern Syria.
Here are five things to understand about ISIS and its fight across the Middle East:
Who They Are: ISIS stands for Islamic State of Iraq and Syria, also known as the Islamic State of Iraq and the Levant (ISIL). The group is trying to form an independent state with territory in Iraq, Syria, and parts of Lebanon. They are led by an Iraqi cleric who goes by the name Abu Bakr al-Baghdadi.
What They Do: ISIS has been fighting against Syrian President Bashar al-Assad as well as other militant Islamic groups in Syria for control of parts of Syria while also fighting the Iraqi government in its quest to form the unified ISIS state. Its brutal tactics have been disavowed as too extreme by al Qaeda's leader Ayman al-Zawahiri.
How Powerful They Are: This week, ISIS forces took Mosul, the third biggest city in Iraq and the biggest one along the Iraq-Syria border. When they took the city, the ISIS seized more than $400 million from city banks, making it richer than many small nations. According to the Associated Press ISIS has also taken effective control of Tikrit, where Saddam Hussein was born.
How They're Able to Fight: Since it controls cities on either side of the Syria-Iraq border, ISIS forces were able to quickly move weapons seized from Mosul into Syria on Monday. The weapons, including Humvees, rifles, missiles and ammunition, will help arm ISIS in its fights on both sides of the border.
The State They Want to Create: The territory in the western part of Iraq that ISIS wants is made up of mostly Sunni Muslims, as is the part of Syria that ISIS wants. Many Sunni Muslims in Iraq have joined ISIS in fighting against the Iraqi Army, which is under the control of a Shiite minority that mainly lives on the other side of the country.