ISIS insurgents fired mortars at the oil refinery in Baiji, forcing the temporary shutdown of Iraq's largest gasoline processing plant. Here's what you need to know about the strategic importance of this refinery. (Davin Coburn and Kate M. Tobey/The Washington Post)

Extremist militants continue their battle with Iraqi security forces in and around a clutch of strategic sites, including the key oil refinery at Baiji. The advance of jihadist group ISIS has shaken the fragile government in Baghdad and may prompt an American military response in a country that was utterly reshaped by the U.S. invasion in 2003.

ISIS's dramatic gains should not have taken anyone by surprise. The terror organization, which began as an al-Qaeda splinter group in Iraq and reemerged amid the tumult of Syria's civil war, has been meticulous in charting its own rise. Last month, the Washington-based Institute for the Study of War (ISW) published an analysis of a number of annual reports released by ISIS that documented the group's attacks, which ranged  from car-bombings to suicide vests exploded to "apostates repented."

(Laris Karklis/The Washington Post) Source: ISW, Long War Journal

In 2013 alone, the group claims to have carried out some 10,000 operations in Iraq, including 1,083 assassinations. Much of its activity has focused on Nineveh province (spelled sometimes as Ninewa). The fall earlier this month of Mosul, Iraq's second city and Nineveh's capital, plunged the country into crisis as ISIS fighters and affiliated militias motored on toward Baghdad.

ISW, using ISIS data
ISW, using ISIS data

ISIS, as its own reports suggest, had diligently built up the capacity to launch this offensive. Charles Lister, a visiting fellow at Brookings Doha Center in Qatar, recounts the backdrop to its campaign:

[ISIS] spent two years breaking senior leaders out of prison and re-establishing a professional command and control structure; expanding operational reach, including into Syria, and exploiting rising Sunni discontent with the Shiite-led government of Iraqi Prime Minister [Nouri] al-Maliki, thereby encouraging sectarianism.

It has expanded extensive underground networks in Sunni strongholds, particularly Mosul, Baghdad and Anbar province; stepped up coordinated, often near-simultaneous bombings; and debilitated Iraqi security force capacity and morale through a concerted campaign of intimidation and assassination.

As a result, the group now commands a virtual state of its own in the Sunni heartlands of Syria and Iraq. Its careful accounting reflects the seriousness and industry behind its scary ambitions. Here's the ISW report on what all the data reported by ISIS indicates:

First, metrics effectively demonstrate the use of centrally distributed resources, such as suicide bombers. Second, metrics provide a higher command with a means to compare subordinate commands and to control main efforts. Third, attack metrics provide a means to communicate organizational efficacy to outside parties, such as donors, al-Qaeda groups, and adversaries.

You can explore more of ISW's analysis on ISIS's annual reports here.