To go along with what Rae was talking about, I'd add that the veterans of the Twilight War might have been fighting for about half a decade and by the end of the war, they are no longer simply fighting for their country and/or a national cause. Those reasons have been ground out of them by the attrition of the war and the bonds they feel with their fellows have been created in those years. New people to the unit will be like strangers to them until the new personnel have had plenty of time to prove themselves to be competent and more importantly, prove themselves to be survivors - after years of war, you might be emotionally hardened against the death of your fellows but it doesn't mean you're going to want to befriend new people.
Then on top of all that, you have the devastation of the warzone that British & US troops will have learnt extends to their homelands - they aren't fighting any more to keep their homes safe, their homes have already been attacked and in some cases, destroyed.
I think in those last few years of the war, the psychological stress on British & US troops would be severe enough given the above factors that replacements to a unit could easily feel alienated enough to want to leave and maybe go home. Others might feel that the war is lost and lost by both sides after they started dropping nukes. Some of them would believe that since both sides have wrecked the planet, there is just nothing left worth fighting for.
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