Grierson was appointed Director of Military Operations at Army Headquarters in 1904. He became involved in simulating potential conflicts, umpiring the Strategic War Game of 1905. In January 1906, during the First Moroccan Crisis, Grierson (DMO) was tasked with drawing up detailed plans for deployment of an expeditionary force to Le Havre in the event of war. He and his deputy Robertson organised a "strategic war game" to explore the options, which persuaded them that British intervention was necessary to avoid French defeat. They began talks with the French General Staff and with the French military attaché Colonel Victor Huguet, and that same year Grierson, Robertson and Huguet toured the Charleroi to Namur area. However, little further progress was made until after Wilson became DMO in 1910.