In fact, I cant think of an occasion when it ever DID succeed ! Far from it, the whole approach is guaranteed to fail, perhaps even designed to fail.
The tell-tale flaw in the construction is in the UN component; most anti- war campaigners talk about "no to war without UN approval". This gives it away; there is virtually no chance the UN could, or even want to avert the disaster. As a conglomerate of world opinion chaparoned by the large miliatry or industrial powers, the UN is, in fcat, simply the wrong body in which to place hopes. By contrast, the campaigners give away their conceits ! What the HELL do they know of the UN ? How come suddenly eveyone is an expert on one of the most complicated organizations in the world, where even those who work IN it have little idea of what really goes on there. (A bit like the EU, or IMF, or the defunct CMEA). Being a member of an NGO and attending a UNO conference may make you FEEL important, but like pornography these gatherings are intended precisely to make the impotent feel important !
No, the UN is on the wordlist of the anti-war campaiagners because its the British state policy to use it for its own purposes thats all. If you want to support Blair's war- drive, go the demo today and talk of the UN !