Conscription wasn't in effect during the Great War. To be honest, I don't think humanity was so doing so bad as it seemed; the outer colonies were falling rapidly of course (deals with the rebel problem nicely) and so did some of the inner colonies, but there was never any blind panic amongst the upper echelons of HIGHCOM and especially ONI. But there was always Infinity in the event that the Covenant won. It takes some of the tension and desperation out of the trilogy to be honest.