Al-Qaeda is an organization of Islamic extremists who advocate jihad, or "holy war" against their enemies, which can include practically anyone who is not a devout Muslim. Radical Islam has historically exhibited extreme hatred toward the United States for a number of reasons, chief among them the long-standing support of the US for Israel. Radical Islamists believe that Jews are "infidels," and that a Jewish state should not exist under any circumstances.

Being without territory or government of its own, in fact being a rogue organization generally, Al-Qaeda and similar organizations resort to terrorist acts to try to get their way. The 9/11 attacks on the World Trade Center towers and the Pentagon were large-scale examples of terrorist methods. But this type of attack is not new. In fact, Al-Qaeda attempted to bring down the World Trade Center towers once before, in 1993. They detonated a truck bomb in the basement below Tower One, seeking to topple it into Tower Two and bring them both down.