The Western Front in WWI was dominated by attempts to break through the German lines, with all sorts of innovations and offensives. Why was never any attempt to flank the quagmire by landing forces behind the lines? The Entente had absolute naval superiority after Jutland, couldn't they have just chosen any stretch of Belgian or German coastline and landed soldiers there, supported by fire from sea?
There were a number of plans for an Allied landing, either on the Belgian coast, or on islands off the German coast. I discussed these plans here. Of these plans, the one that was closest to coming to fruition was Operation Hush, a plan to land a division, supported by nine tanks, on the Belgian coast west of Ostende. This was ready to go from July to October 1917, but was cancelled thanks to the failure of the Third Battle of Ypres. I have written a post about it here.