I've been thinking about the most important few months of the 20th Century again; August-November 1914. I'm not a believer in the Schlieffen plan's ability to defeat the French in one fell swoop, but there were opportunities to give the Germans a better strategic offensive-tactical defensive position in France. I think it was entirely possible for the Germans to hold their position on the Marne and win the Race to the Sea and hold the French channel coast down to Dieppe.
Ad. Bacon of the WW1 Dover patrol stated that if through-channel shipping and direct food imports to London itself stopped a third of london's population would have to be evacuated to parts of the country where they could be more easily fed.
Similarly if the front lines held on their Sept 9th (ish) position Paris and surrounds would be within range of artillery fire from long-range guns (rail guns and the like). I think this would cause a partial evacuation of Paris as well.
So what would be the effects on the war if both Paris and London had large portions of their populations evacuated for a period of years? Would the public continue to support the war effort if millions of essential war-workers were deprived of their families, millions more had evacuees billeted on them, not to mention the evacuees themselves?