Richard over at Le Fleur de Lys too has an excellent post on the war experiences of J.R.R Tolkien. Follow the link to have a read. One thing the post elucidates is the obviously reminiscent scenes of the crossing of the Dead Marshes to what must have been the hideously bleak, stinking and foul scene of the Trenches, vast and hopeless. C.S Lewis anticipated this view long before Tolkien's daughter Priscilla did though.
Be on your guard though; Tolkien repudiated any allegorical reading of The Lord of the Rings. Just as Mordor did not represent Russia, Sauron Stalin or the Orcs Communists, neither does the crossing of the marshes ''represent'' the Trenches. Naturally, as C.S Lewis points out, there are similarities, but these are to be taken only as reflecting how the real life experiences affected the imagination of the author, a man deeply moved by the death of his close friends while still in his youth. Incidentally, Christopher Tolkien, the editor of much of Tolkien's posthumously published work, was named after Christopher Wiseman.