More on Bush's reasons from the article:

By invading Iraq, Bush has taken over the Iraqi oil fields, and persuaded the UN to lift production limits imposed after the Kuwait war. Production may rise to 3m barrels a day by year end, about double 2002 levels. More oil should bring down Opec-led prices, and if Iraqi oil production rose to 6m barrels a day, Bush could even attack the Opec oil-pricing cartel.

Control over Iraqi oil should improve security of supplies to the US, and possibly the UK, with the development and exploration contracts between Saddam and China, France, India, Indonesia and Russia being set aside in favour of US and possibly British companies. And a US military presence in Iraq is an insurance policy against any extremists in Iran and Saudi Arabia.

Overseeing Iraqi oil supplies, and maybe soon supplies from other Gulf countries, would enable the US to use oil as power. In 1990, the then oil man, Dick Cheney, wrote that: "Whoever controls the flow of Persian Gulf oil has a stranglehold not only on our economy but also on the other countries of the world as well."

In the 70s, the US agreed with Saudi Arabia that Opec oil should be traded in dollars. American governments have since been able to print dollars to cover huge trading deficits, with the further benefit of those dollars being placed in the US money markets. In return, the US allowed the Opec countries to operate a production and pricing cartel.

The rest talks about Blair's reasons...Guardian