ISIS literally is al qaeda. its right there. read the fucking wiki article. i put important parts in bold in case you actually have trouble reading
Following the 2003 invasion of Iraq, the Jordanian Salafi jihadist Abu Musab al-Zarqawi and his militant group Jama'at al-Tawhid wal-Jihad, founded in 1999, achieved notoriety in the early stages of the Iraqi insurgency for their suicide attacks on Shia mosques, civilians, Iraqi government institutions and Italian soldiers partaking in the US-led 'Multi-National Force'. Al-Zarqawi's group officially pledged allegiance to Osama bin Laden's al-Qaeda network in October 2004, changing its name to Tanzim Qaidat al-Jihad fi Bilad al-Rafidayn (تنظيم قاعدة الجهاد في بلاد الرافدين, "Organisation of Jihad's Base in Mesopotamia"), also known as al-Qaeda in Iraq (AQI).
In January 2006, AQI joined with several smaller Iraqi insurgent groups under an umbrella organisation called the Mujahideen Shura Council (MSC). According to counterterrorism researcher Brian Fishman, the merger was an attempt to give the group a more Iraqi flavour, and perhaps to distance al-Qaeda from some of al-Zarqawi's tactical errors, such as the 2005 bombings by AQI of three hotels in Amman. On 7 June 2006, a US airstrike killed al-Zarqawi, who was succeeded as leader of the group by the Egyptian militant Abu Ayyub al-Masri.
On 12 October 2006, the Mujahideen Shura Council united with three smaller groups and six Sunni tribes to form the "Mutayibeen Coalition". It pledged "To rid Sunnis from the oppression of the rejectionists (Shi'ite Muslims) and the crusader occupiers ... to restore rights even at the price of our own lives ... to make Allah's word supreme in the world, and to restore the glory of Islam". A day later, the Mujahideen Shura Council declared the establishment of the Islamic State of Iraq (ISI), comprising Iraq's six mostly Sunni Arab governorates. Abu Omar al-Baghdadi was announced as its emir, and al-Masri was given the title of Minister of War within the ISI's ten-member cabinet.
In late 2009, the commander of US forces in Iraq, General Ray Odierno, stated that the ISI "has transformed significantly in the last two years. What once was dominated by foreign individuals has now become more and more dominated by Iraqi citizens". On 18 April 2010, the ISI's two top leaders, Abu Ayyub al-Masri and Abu Omar al-Baghdadi, were killed in a joint US-Iraqi raid near Tikrit. In a press conference in June 2010, General Odierno reported that 80% of the ISI's top 42 leaders, including recruiters and financiers, had been killed or captured, with only eight remaining at large. He said that they had been cut off from al-Qaeda's leadership in Pakistan.
On 16 May 2010, Abu Bakr al-Baghdadi was appointed the new leader of the Islamic State of Iraq.
On 8 April 2013, al-Baghdadi released an audio statement in which he announced that the al-Nusra Front had been established, financed, and supported by the Islamic State of Iraq, and that the two groups were merging under the name "Islamic State of Iraq and Al-Sham"
conclusion: ISIS used to be al qaeda and the second american invasion in iraq caused iraqi people to join ISIS. that the first american invasion resulted in al qaeda gaining a foothold in iraq, you can read that in the article about the first gulf war