Of these battle areas of WWI -- Marne, Verdun, and Meuse-Argonne -- I think only one of these is on the Western front. Is this correct?
No, this is not correct. All of the areas that you mention in this question are in fact on the Western Front.
What you must remember is that the term "Western Front" refers to all of the fighting that went on in World War I in Western Europe. This means that it refers to all of the battles in Belgium and in France. All of the areas that you mention are in France. Therefore, all of them are part of the Western Front.
The Marne was the site of two major battles in WWI, one in 1914 and one in 1918. Verdun was the site of a huge battle in 1916. The Meuse-Argonne area was site of the final allied offensive in the war in 1918. Americans fought there and at the Second Battle of the Marne.
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