The U.S. Took Years to Establish Control Over Iraq. It's Taking ISIS Six Months.

An extremist group has been marching toward Baghdad, relatively unopposed.

Kurdish Iraqi Peshmerga forces deploy on the outskirts of Kirkuk, on the main road between Kirkuk and Mosul in northern Iraq. (National Journal)

It took the U.S. years to establish control over Iraq. Now, an extremist group that is too violent even for al-Qaida has taken over some of the country's most important cities within a span of days.

The Islamic State of Iraq and Syria, a Sunni militant group, has taken control of some of the largest cities in Iraq this week, arriving within striking distance of the capital city of Baghdad. And they've only been on the offensive path for the last six months.

On Thursday, the extremist group took Tikrit, Saddam Hussein's hometown, located just over a two hours' drive north of Baghdad. Fighting was reported in Samarra, a city just 80 miles north of Baghdad, and in Abu Ghraib, located just outside the capital.

Thursday's victory for ISIS comes just a day after the group took Mosul, the second largest city in Iraq. Fallujah and Ramadi, the two largest cities in the embattled Anbar province, have been largely under ISIS control since January.

Just how quickly has this all happened? Compare it to how long it took the United States to gain control over these cities in the mid-aughts.

Tuesday saw ISIS overrun Mosul in one night as security forces fled the city. The coalition campaign to take the city in 2004 took over a week and failed to stabilize the city.

Fallujah fell to ISIS in five days. It took coalition forces nine months to regain control of the city in 2004 after the March ambush that killed four Blackwater contractors.

The coalition campaign to retake Ramadi in 2006 lasted five months and ended in an inconclusive victory; ISIS took large parts of it in the span of a week earlier this year, although its hold over the city has been tenuous.

Now, Baghdad is well within the grasp of ISIS. "We are using the word 'encircle,' " Jessica Lewis, an expert on ISIS at the Institute for the Study of War, told Time. "They have shadow governments in and around Baghdad, and they have an aspirational goal to govern."