Colonel Gaddafi has been killed in Libya after 42 years of a brutal dictatorship. The Libyan people will be celebrating tonight but if, as appears likely, he was killed after suffering wounds from a Nato aircraft it raises questions about the legality of the bombing campaign in Libya.

When the war began way back in March much of the debate was whether the UN Resolution 1973, which authorised in paragraphs four and eight, "all necessary measures" to protect civilians in Libya, covered attacks on Gaddafi and senior figures in the regime. In late March the now-former British defence minister Liam Fox claimed that Gaddafi was a "legitimate target" for assassination within the remit of 1973.

Within days Nato stressed that was not their position and David Cameron was quick to say he disagreed with Fox. The Chief of the Defence Staff General Sir David Richards went even further: "Absolutely not. It is not allowed under the UN resolution and it is not something I want to discuss any further."

Yet as the war has continued it has become increasingly clear that Nato has accepted Fox's interpretation of 1973, despite publicly claiming the opposite. First there were attacks on his command and control centres in Tripoli that aimed, according to Fox, to increase "psychological pressure" of the Gaddafi family. Then there were attacks on his family home, which allegedly killed his youngest son Saif al-Arab Gaddafi and children. Since the fall of Tripoli the realistic threat to civilians has fallen significantly and while the pursuit of Gaddafi by the Libyans is legitimate, the continued use of Nato air force hints at regime change.

If Nato stepped over the line between protecting civilians and assassination attempts it is unclear exactly when this happened. International law lags behind real politik, particularly in the regulation of unmanned surveillance drones, which have been used in Libya. International law regulating assassination often ends up falling back on the Hague Convention of 1899 which confirms that it is illegal "to kill or wound treacherously individuals belonging to the hostile nation or army."

But, as Kristen Eichensehr has argued, "in recent years, and especially since September 11... the United States [and others] have reframed [assassinations]as "targeted killings," defining the victims as "enemy combatants" who are therefore legitimate targets wherever they are found."

Over the course of the war Nato has gradually redefined its goals, to the extent that David Cameron is hailing the murder of a man as a victory for democracy. The lines between liberal interventionism and regime change have blurred so fast it is difficult to know whether they have strayed beyond the lines of Resolution 1973.

Policy-makers will rejoice that Gaddafi is dead as it vindicates the theory of quick and easy liberal wars. Yet if he has been killed by bombs flown by Nato airplanes it finally removes the illusion that this is a Libyan revolution. Even if he wasn't it is fairly clear that Nato reconnaissance led the Libyan rebels to him. The purpose of going to war was to protect the population of Libya. Was the killing of a deposed leader legitimate within that framework? When David Richards said "absolutely not", perhaps what he really meant was "quite possibly".

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