Since you're apparently too stupid to look it up, I Googled it for you:
Following the Soviet Union's withdrawal from Afghanistan in February 1989, bin Laden returned to Saudi Arabia. The Iraqi invasion of Kuwait in August 1990 had put the Kingdom and its ruling House of Saud at risk. The world's most valuable oil fields were within easy striking distance of Iraqi forces in Kuwait, and Saddam's call to pan-Arab/Islamism could potentially rally internal dissent.
In the face of a seemingly massive Iraqi military presence, Saudi Arabia's own forces were well armed but far outnumbered. Bin Laden offered the services of his mujahideen to King Fahd to protect Saudi Arabia from the Iraqi army. The Saudi monarch refused bin Laden's offer, opting instead to allow US and allied forces to deploy troops into Saudi territory
The deployment angered bin Laden, as he believed the presence of foreign troops in the "land of the two mosques" (Mecca and Medina) profaned sacred soil. After speaking publicly against the Saudi government for harboring American troops, he was banished and forced to live in exile in Sudan.
At the same time, al-Qaeda ideologues instructed the network's recruiters to look for Jihadi international Muslims who believed that extremist-jihad must be fought on a global level. The concept of a "global Salafi jihad" had been around since at least the early 1980s. Several groups had formed for the explicit purpose of driving non-Muslims out of every Muslim land at the same time and with maximum carnage.
In 1996, al-Qaeda announced its jihad to expel foreign troops and interests from what they considered Islamic lands. Bin Laden issued a fatwa (binding religious edict),[153] which amounted to a public declaration of war against the US and its allies, and began to refocus al-Qaeda's resources on large-scale, propagandist strikes.
On February 23, 1998, bin Laden and Ayman al-Zawahiri, a leader of Egyptian Islamic Jihad, along with three other Islamist leaders, co-signed and issued a fatwa calling on Muslims to kill Americans and their allies where they can, when they can.[154] Under the banner of the World Islamic Front for Combat Against the Jews and Crusaders, they declared: "The ruling to kill the Americans and their allies – civilians and military – is an individual duty for every Muslim who can do it in any country in which it is possible to do it, in order to liberate the al-Aqsa Mosque and the holy mosque from their grip, and in order for their armies to move out of all the lands of Islam, defeated and unable to threaten any Muslim. This is in accordance with the words of Almighty Allah, 'and fight the pagans all together as they fight you all together,' and 'fight them until there is no more tumult or oppression, and there prevail justice and faith in Allah'."
No need to thank me. Let me know the next time you can't figure out how to look something up.
Poor Blabo.
https://en.wikipedia.org/wiki/Al-Qaeda#Gulf_War_and_the_start_of_US_enmity