On August 30th of 1918, Colonel Arthur L. Conger of the American Expeditionary Force had shut himself in his hotel room. There, in privacy, he began typing a planned offensive by United States troops in Belfort, a city in France near the German border. Belfort was, at the time, considered an important enough location to stage a major Allied attack.
In those days, they had carbon paper, where you slid a piece of inked paper between two sheets of white paper to make a “carbon copy.” Conger typed up the plan, then folded the original and the copy, and placed them in his coat pocket. He carelessly discarded the crumpled carbon paper in a wastebasket near where he sat. After that, he went out for a walk around the grounds of the hotel. When Colonel Arthur Conger returned to his hotel room, the carbon paper had mysteriously disappeared from the wastebasket.
The thing is, the battle plan was false, and was only intended to deter troops away from the real scene of the action. The Germans were supposed to believe that Belfort would be a perfect launching pad for an enemy attack on Germany. United States Army General John J. Pershing's actual plan was to attack Saint Mihiel, a region that was then occupied by the Germans.
Some French officers believed that the Americans were too unsophisticated against the modern European warfare of the French. So it meant a lot to the American army to do well in St. Mihiel. It was the first significant American led offensive on the western front. It was important, yet difficult, if not impossible, to keep the plan of attack a secret. Replacing French forces with American troops involved the moving of 600,000 soldiers.
This series of events, later termed “The Belfort Ruse,” was based on a suggestion of General Henri Philippe Petain, Commander of the French armies. Petain had penned a letter to General Pershing, suggesting that the Germans be misled as to where the U.S. offense would begin, as it seemed that Pershing's plans were common knowledge in Paris.
Congor, who had nothing to do with the planning of the ruse, was (probably) sent to that particular hotel because American intelligence had gathered that a German agent was staying there. It was hoped that the scheme would cause the Germans to relax their hold on St. Mihiel, and send more soldiers over to Belfort. Only four Americans knew of the ruse: General Pershing, Colonel Conger, McAndrew, and Brigadier General Fox Connor.
The French soon started spreading rumors that the Americans would attack at Belfort. Undoubtedly, soldiers were detached to both locations.
It is not clear how well the ruse worked. What happened was, despite the carefully considered judgment of the Germans, Ludendorff reinforced the Mihiel salient, as well as recruiting many soldiers in reserve, who would fight along both sides of the river if the need should arise. It seems obvious that the German army had plenty of soldiers.