What lay behind the Libya intervention?

Since the start of the Franco-British intervention in Libya, which on British and American insistence last March was turned into a NATO affair, some commentaries on the left have interpreted the action as Western imperialism. It was called an effort to seize control of Libya's enormous oil reserves, in the guise of humanitarian intervention. Although I am willing-more willing than most-to think the worst of the motivations of states, I find it hard to see why the Western countries would want an expensive war to seize the oil to which they already had ample access through purchase on the international market. Adapted from the source document.