The expansion of the Islamic State of Iraq and the Levant (ISIL) into Iraqi cities has been one of the most eye-catching events in the Middle East this week.
It all started with the capture of Mosul, and expanded to Tikrit, the hometown of executed dictator Saddam Hussein.
ISIL spokesman Abu Mohammed Al-Adnani promised the battle would "rage" in the capital Baghdad as well as Karbala - one of the holiest cities for Shia Muslims.
On Twitter, ISIL said it would "not stop this series of blessed invasions" that witnessed the fall of Nineveh province in the north and swathes of Kirkuk and Saleheddin provinces further south.
On 30 December, security forces cleared a year-long Sunni protest camp, marking the beginning of ISIL's growing role in Iraq.
Such government action caused the eruption of fighting near Ramadi and spread to Fallujah west of Baghdad. ISIL is currently imposing its grip on parts of the two cities.
It was the first time anti-government fighters have exercised open control since the height of a rebellion against the Shia government of Nuri Al-Maliki.
ISIL's leadership & size
Being an offshoot of Al-Qaeda, ISIL - known in Arabic as Daesh - adopts a jihadist approach to achieving the goal of creating an Islamic caliphate.
In fact, ISIL is taking advantage of the situation in war-torn Syria and executes operations in both countries.
Formed in 2013, many reports suggest that ISIL developed from within Al-Qaeda in Iraq - which rose against the US invasion of the country in 2003, though Al-Qaeda denies connections with the group.
The militant group comprises of thousands of fighters under its umbrella. However, little is known about their identities. Moreover, the existence of foreigners among the ISIL's ranks is highly questioned.
ISIL's leader is Abu Bakr Al-Baghdadi. According to a BBC report, it is believed that he was born in Samarra north of Baghdad in 1971 and joined the anti-US insurgency during the invasion's aftermath.
Many videos and pictures have been posted on the internet of extremely brutal acts, such as murder and torture, committed by members of the ISIL against civilians in Iraq and Syria.
Determining backers of the ISIL - especially in terms of finance and weaponry - signify another part of the ambiguity surrounding the rapid rise of the militia.
Bigger impact on Syria
Syria's ongoing conflict works as follows: Bashar Al-Assad regime is fighting against rebels.
But the entrance of Islamist rebels had further complicated the situation. ISIL came into conflict with regime forces and the Free Syrian Army (FSA) - Syria's moderate rebel forces on the ground - at the same time.
Al-Baghdadi's fighters went even further by clashing with Al-Qaeda fighters in Syria under Ayman Al-Zawahiri, Osama bin Laden's successor.
On 9 April 2013, Al-Baghdadi declared a merger between his group and Al-Nusra. Al-Nusra rejected the merger, renewing its pledge of allegiance to Al-Zawahiri.
Al-Nusra's refusal created a major split in the global jihadist movement, creating a clash over hegemony between these factions in Syria.
In January, Al-Nusra lost control of the northern city of Raqqa to ISIL fighters, Reuters said.
Fighting is still taking place over the oil-rich, eastern province of Deir Al-Zor, which borders Iraq's Anbar province, a majority Sunni region where ISIL has fought against Iraqi forces.
Consolidating control of Deir Al-Zor would give ISIL an international border and territory stretching 500 miles from the centre of Iraq across to the Mediterranean and the border with Turkey.