Islamic State, which has claimed the deadly attacks in Brussels, started as a small terrorist group in the late 1990s and came to prominence around the time of the 2003 US-led invasion of Iraq.
The bombings in Belgium are the latest the extremists claimed in Europe and reaffirm that the group's potential to cause harm in otherwise peaceful locations. The attacks in Paris last year were their deadliest outside of the Islamic world.
The United States organized an international anti-Islamic State coalition in 2014 after the radical group seized Mosul, Iraq's second city, and declared itself the modern-day caliphate to which all Muslims owe allegiance. The group also appeared set to massacre and enslave the country's Yezidi religious minority and aimed to advance further.
The coalition, with the aid of Kurdish and Iraqi forces on the ground in Syria and Iraq respectively, has begun to take back chunks of territory from Islamic State, but it is far from defeated. Meanwhile, it has set up wings in other countries, such as chaotic Libya.
Many top Islamic scholars and community leaders have thoroughly denounced the organization.
However, it continues to mesmerize disenfranchised Muslim men and women, especially youths, who see through it a path to salvation. Furthermore, it attracts talented individuals, from military commanders to doctors.
Islamic State's slogan, often translated as "lasting and expanding," hints at its ambitions, to hold territory and branch outward, trying to mimic the early achievements of the Prophet Mohammed and his followers.
The group is primarily based out of Syria and Iraq, but has a number of affiliates who control pockets of land from south-east Asia to West Africa, often taking advantage of anarchic situations in areas populated by Sunni Muslims.
The extremist Sunni group, which once informally held the name al-Qaeda in Iraq, was originally headed by Jordanian-born Abu Musab al-Zarqawi, who had previously spent time with jihadists in Afghanistan.
He led a murderous campaign in Iraq, intentionally stoking sectarian strife between the country's Sunni and Shiite Muslims, helping to provoke the civil war after the fall of Saddam Hussein.
Osama bin Laden and other al-Qaeda leaders, from the onset, expressed concern about al-Zarqawi killing fellow Muslims.
Moreover, at a theological level, al-Qaeda supported the creation of a caliphate, but felt this was a far-off dream. Al-Zarqawi's gang insisted the right time is the present day.
Already in 2005, al-Zarqawi organized suicide attacks in Jordan, killing dozens, including guests at a wedding ceremony. This indicated not only a disregard for civilian lives, but a direct intention to kill the innocent in any country, including Muslims.
A US airstrike north of Baghdad eight months later killed al-Zarqawi. His group, which regularly changed names, faced a serious decline in the following years.
However, its fortunes were reversed under a new leadership in 2010, including the current chief, or self-styled caliph, Abu Bakr al-Baghdadi.
The Syrian civil war acted as a rallying call for the group, which claimed it would overthrow President Bashar al-Assad, who it described as a despot and denounced with sectarian language, as he is an Alawite, a Muslim minority sect.
In 2014, the group, the so-called Islamic State in Iraq and the Levant, formally split with al-Qaeda and later in the year rebranded itself, dropping the extra words and declaring itself the one true Islamic State to which all Muslims must immigrate.
Islamic State espouses a tough "with-us-or-against-us" ideology, denouncing anyone not fully on board as a disbeliever.
It uses ultra violence and slick media productions depicting its acts of horror, such as beheadings, not only to scare enemies but to recruit new followers.
Among its followers are European Muslims who counter-extremism experts say feel disenfranchised and out of place, especially in poor, urban areas.