His was described as a mission impossible pretty much from the start. Would the Nobel laureate whose career had weathered some spectacular lapses in international peacekeeping – the Rwandan genocide, the Srebrenica massacre, Somalia and Darfur – succeed with 300 unarmed observers in policing a ceasefire in Syria neither side had an interest in observing? The answer came on Thursday when Kofi Annan resigned from his role as the special envoy for the UN and the Arab League. Try as one might to concoct a scenario where the six-point Annan plan for a negotiated political transition may one day be resuscitated – a successor will be found as envoy and, as Annan says, the plan is the security council's, not his – it is hard to avoid the conclusion that his resignation marks the end of diplomacy in Syria. It is a landmark moment.

Some argued almost from the start that Annan, one of the world's most seasoned diplomats, only made things worse. The means, in the form of a ceasefire, offered loyalist troops a respite, while the end ,a political process became a free get-of-jail-card for Bashar Assad. The Annan plan, it was said, exemplified the gap between expectation and delivery that Assad exploited with murderous intent. Diplomacy lurching in bad faith from one capital to another was never going to be a match for fast-flowing events on the ground. And it is true that the Assad regime never implemented the peace plan, in spite of a pledge on 27 March to do so – although a ceasefire on 12 April did stop the government's shelling of civilian communities temporarily. But neither did the opposition ever seriously consider laying down their arms. The undoing of the Annan plan surely lay not so much with the plan itself – there never was any other real alternative to stop a brutally repressed civilian insurrection turning into a civil war – but in the UN security council that approved it.

In his parting shot, Annan named international division, the support for a proxy agenda, and the fuelling of violent competition on the ground as the three factors that torpedoed his efforts. While Russia, China and Iran failed to realise that Syria's leadership had lost all legitimacy, and thus failed to put any pressure on Assad to go – Annan said quite bluntly that Assad had to go "sooner or later" – the US, UK, France, Turkey, Saudi Arabia and Qatar all failed, in his view, to press the opposition into anything that could go under the name of a political process. It is not just that the opposition were fractured. It was that they failed to accept any element of the existing regime as part of a transitional process.

Collectively, the competing blocks in the security council were feeding the beast their appointed envoy was trying to tame. It is wholly redundant for David Cameron to call as he did after his talks with Vladimir Putin in London for tougher UN resolutions when Russia has already used its security council veto three times to block any UN action.

Annan's is as damning an indictment of the countries supplying and supporting the opposition with arms and training as it is on Russia. His departure means that the war stokers will now have to live with what they wished for. And if that is an armed resolution of the conflict that has already cost 20,000 lives, they will be getting off lightly. There may be no military resolution at all.

Assad's forces are undoubtedly weaker than they were one month ago, depleted by the fighting, defections and the growing military strength of the opposition. Aleppo, with its narrow streets and open rebel supply lines, presents a different prospect to the Syrian army than Damascus. Lacking boots on the ground to recapture the city, the army will use overwhelming force against areas where the insurgents are based. But none of this amounts to a clinical end to the bloodshed. Annan's departure as international envoy has not led to any soul-searching. Rather Reuters reports that Barack Obama has secretly authorised CIA backup for the Free Syrian Army. Everyone is digging in for a longer and bloodier war.