When World War I began in August 1914, many people anticipated that the war would end by Christmas of that year. Which event extended the war into 1915 and beyond?

The French stopped the German advance at the First Battle of the Marne in September 1914. This prevented Germany from quickly defeating France, as its plan had called for, and led to a prolonged stalemate and trench warfare on the Western Front. The Americans entered the war later, in 1917, so their entry did not extend the conflict into 1915.

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