The latest Joint Forces Quarterly has an interesting article from retired Brigadier General Jeffery E. Marshall regarding "assumptions" in military planning models, always seemingly poised to thwart even the best-laid plans.
I've long argued that discussion of the Military Decision-Making Process (MDMP) ought to be balanced with a case study of the inner workings of the German and French general staffs in the run-up to the First World War. Fortunately, Brig. Gen. Marshall briefly touches upon the German Schlieffen Plan (though not covering its counterpart in the French Army, Plan XVII). According to Brig. Gen. Marshall:
History is replete with examples of assumptions that were neither tested and validated nor balanced with a branch plan to execute if the assumptions proved incorrect. For example, in World War I, the German Schlieffen Plan assumed that the British would not intervene and that the French could be defeated in 6 weeks. The Germans were wrong on both counts. The British intervened, the French held on, and a bloodbath ensued.
An interesting example, but certainly not the most foolhardy assumption on the part of Moltke and the German general staff. The entire Schlieffen plan hinged on the assumption that German troops could overrun France and force a surrender before the Russian Empire could mobilize and strike at Germany. Expecting a swift victory, as in the Franco-Prussian War of nearly forty years earlier, Schlieffen and his successor, Moltke the Younger, planned to first defeat France, and then swiftly divert their massive armies to the Eastern front to halt the Russian offensive.
Unfortunately, the Russians did mobilize more quickly than anticipated, forcing Moltke to divert two corps from the Belgium to Prussia. Though the move allowed the Germans to halt the Russian offensive at Tannenburg, in East Prussia, it was not without ultimate failure at the Marne (might the two corps have turned the tide?), and the expedient ruin of the Austrian-Hungarian army.
While properly examining this assumption might have averted failure on the part of the Germans during the First World War, the chaos brought about by the intricacies of international politics, especially during wartime, makes proper planning all but impossible at the strategic level. Neither side could have envisioned the turn of events which led to the Ottoman Empire's involvement on the side of the Axis Powers, made possible by the escape of two German capital ships into the Dardanelles during the first few days of the war.
Planning paradigms such as the Military Decision Making Process may work well when a desired end-state is certain, and within a relatively closed system (such as a large exercise at a combat training center, or for a movement of equipment from one point to the next), but it's hardly applicable at the strategic to grand strategic level. Learn from the master, General James Mattis:
Staffs have been seen to often apply these processes mechanistically; as if progressing through a sequence of planning steps would produce a solution. I would expect this habit to be common particularly in organizations reacts to these processes rather than leads them. "Over-proceduralization" inhibits the commander and staff's critical thinking and creativity, which are essential for finding a timely solution to complex problems...our current doctrinal approach to fostering clear, careful thinking and creativity, particularly early in design and planning, is insufficient and ineffective.