The war in Libya is about a lot more than Muammer Gaddafi. Its outcome will reverberate around the Middle East and will affect international politics for decades. A vital principle is at stake.

The supporters of outside intervention believe that they are battling not just to stop atrocities in Libya itself, but to lay down a marker for the future. They want to show that the age when a dictator could massacre his own citizens is coming to a close. Bernard Henri-Lévy, a French philosopher who played an improbable role as a link between the Libyan rebels and President Nicolas Sarkozy, has said: “What is important in this affair is that the ‘duty to intervene’ has been recognised.”

Nicholas Kristof, writing in The New York Times, makes a similar point – “World powers have the right and obligation to intervene when a dictator devours his people.” This idea was approved by the UN in 2005 and, according to Mr Kristof, the Libyan intervention is “putting teeth into that fledgling concept”.