The connection that Western intelligence agencies fear to make.

The conventional wisdom was these two are completely different types of people because Iran is Shi’a, the al-Qaeda people are Sunni, and therefore, you know, the two would never mix. What happened in the end was that they did because they both had a common interest” in fighting the United States.
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The Iranian regime created Hezbollah in 1982 to serve as its terrorist proxy army throughout the world. Hezbollah and al-Qaeda have collaborated for many years. The Saudi branch of Hezbollah helped al-Qaeda commit the deadly Khobar Towers attack in June 1996. The 9/11 Commission elaborates:

In late 1991 or 1992, discussions in Sudan between al-Qaeda and Iranian operatives led to an informal agreement to cooperate in providing support — even if only training — for actions carried out primarily against Israel and the United States. Not long afterward, senior al-Qaeda operatives and trainers traveled to Iran to receive training in explosives. In the fall of 1993, another such delegation went to the Bekaa Valley in Lebanon for further training in explosives as well as in intelligence and security. Bin Laden reportedly showed particular interest in learning how to use truck bombs such as the one that had killed 241 U.S. Marines in Lebanon in 1983. The relationship between al-Qaeda and Iran demonstrated that Sunni-Shi’a divisions did not necessarily pose an insurmountable barrier to cooperation in terrorist operations (emphasis added).

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