From
Prospect, a British current affairs magazine. It's a long but excellent summary.
Five years on, everyone has a theory about the real causes of 9/11. They range from the nutty (it was the US government) to the plausible but flawed (a response to foreign occupation) to the credible (collateral damage from a clash within Islam)
No event in recent times has produced as many explanations as the 11th September attacks five years ago. Within the space of an hour, al Qaeda inflicted more direct damage on the US than the Soviet Union had done throughout the cold war, a cataclysm seen by more people than any other event in history. Yet it took only 19 men armed with small knives to destroy the World Trade Centre, demolish a wing of the Pentagon and kill 3,000 people. This mismatch has led some—especially in the Muslim world—to seek a deus ex machina to explain what otherwise appears inexplicable. The usual suspects have been assembled on 9/11's grassy knoll: the Jews were behind the attacks; the US government engineered them; the "Cheney-Bush energy junta" planned them so that they could grab the oil fields of central Asia, and so on.
Osama bin Laden himself claims that al Qaeda was solely responsible for 9/11. In 2004, he released a video in which he explained his dealings with lead hijacker Mohammed Atta. After the largest criminal investigation in history, the US government's 9/11 commission also concluded that al Qaeda was solely responsible for the attacks.
Attributing the sole responsibility for 9/11 to al Qaeda then brings us to the larger question: what caused al Qaeda to launch the attacks? Explanations for the attacks can be sorted into two categories—the seemingly plausible but flawed, and the more credible.
Plausible but flawed theories
Poverty. Many politicians and commentators see the poverty of the middle east as a factor. (Some political leaders even argued that the Doha round of trade talks, launched soon after 9/11, were intended partly to quash terrorism.) This claim is not supported by the evidence. Those who attacked on 9/11 were sons of the middle eastern middle and upper class, not the dispossessed. Throughout recent history, from the Russian anarchists to the German Baader-Meinhof gang in the 1970s, terrorism has largely been a bourgeois endeavour. Al Qaeda is no different.
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More:
http://www.prospect-magazine.co.uk/article_details.php?id=7717