The traditional definition of "left opportunism," of which anarchism is one variety," is an overestimation of the power of the ruling class, an underestimation of the power of the working class. That's schematic and hardly applicable to concrete conditions without careful analysis. But it fit Weatherman perfectly. A young woman I had recruited to SDS and socialism, rode down to an SDS conference in Texas with a bunch of Chicago Weatherman people (and I financed it). She came back totally committed to them. At one point she explained that the only way the u.s. could achieve socialism was if we were occupied by the Red Army, since the u.s. working class was too racist ever to build socialism. Lenin's observation was that anarchism was the penalty paid by the working class for its sins of opportunism. This certainly fits Chuck O and Thomas S.: the anti-war movement has been to passive, hence let's go smash windows (in despair masquerading as militancy).
But left-opportunism isn't always manifested by greater militancy. Though the SWP people (from the '60s) I know vigorously deny this, it has always seemed to me that their insistence on large _single-issue_ peaceful demos, AND ONLY THAT, also was grounded in distrust of the working class: that is, bourgeois ideology was so strong that discussion of socialism (and of other complex issues) had to be confined within The Party. Discussion and political theory within the larger mass movement was to be confined to tactics of building those demos.
Within the same schematism, the UfPJ and groups associated with it can be seen as right opportunist -- they overestimate the power of the working class (or, if you will, of "The People"), and hence do not realize that those who sup with the Devil (the DP) need to carry a long spoon. Hence their participation over the last year in absorbing the anti-war movement into the DP, where it dies.
Carrol
p.s. The quote from Stalin is interesting in that Mao in one of his essays, after emphasizing the need for correct policy and skilled cadre, _then_ emphasized that sometimes it wasn't due to subjective factors that the movement lost: sometimes the enemy is simply stronger. Of course Stalin's proposition does provide the rationale for shooting everyone involved in a losing battle.