It originated in 1999
According to Wikipedia The group originated as Jama’at al-Tawhid wal-Jihad in 1999, which pledged allegiance to al-Qaeda in 2004. The group was formed by the Salafi jihadist Abu Musab al-Zarqawi.
Al-Qaeda in Iraq (AQI) was formed
Abu Musab al-Zarqawi pledged alliance to Al-Qaeda and founded al-Qaeda in Iraq (AQI). The group had the goal of attacking US targets and their allies as a result of the invasion of Iraq in 2003.
The group struggled to seize power in Iraq
The group planned to seize power in the central and western areas of Iraq and turn it into a Sunni caliphate but their efforts failed.
Zarqawi’s successors rebranded AQI as ISI
After Zarqawi was killed his successors rebranded AQI as ISI , Islamic state in Iraq. Later the name was changed to ISIS or the Islamic state in Iraq and Syria.
They branched out of al-Qaida
ISIS branched out of al-Qaida after they took over large areas of Iraq and Syria. Al-Qaida has announced that ISIS doesn’t represent them.
They grew during the Syrian war
Al-Baghdadi began sending Syrian and Iraqi ISI members experienced in guerilla warfare across the border into Syria to establish an organisation there. Led by a Syrian known as Abu Muhammad al-Julani, this group began to recruit fighters and establish cells throughout the country. (See Summary of the Syrian Crisis)
The US gave weapons to Syrian Rebels
The US gave weapons to Syrian Rebels as they wanted to help them end the rule of Al-Assad because it’s believed to be the strategic depth of the Shia expansion (Iraq and Iran). Many of those rebels joined ISIS later. (See How Did the Americans Win the Revolutionary War?)
It’s believed that ISIS grew out of US support
According to a declassified secret US intelligence report, written in August 2012, the US had interests in allowing a Salafist principality to be established in the area to help stop Al-Assad.
It’s believed that some gulf countries are involved
It’s believed that some Gulf countries are also involved in the creation of ISIS.
Many Syrian rebels joined ISIS
Many Syrian rebels joined ISIS and fought along them.
The Isolation of Sunnis helped ISIS
After the invasion of Iraq by the US Sunni Muslims were sidelined from politics. This created a gap that ISIS , who is a Sunni group, managed to exploit.
Mercenaries joined ISIS
ISIS took over oil fields and had enough money to pay Mercenaries.
How ISIS was formed?
Here’s how ISIS was really founded | An Overview of ISIS – Islamic State of Iraq and Syria