Final answer:
The key lesson from World War I that was not learned by French and British leaders was the significance of armored warfare. Despite the introduction of tanks in World War I, their strategic significance was not fully appreciated until later conflicts.
Step-by-step explanation:
The one lesson of World War I that the French and British leaders had not learned was b) The significance of armored warfare. World War I marked the introduction of new military technology including tanks, machine guns, long-range artillery, and airplanes, but the effective use of these innovations, such as armored warfare, was not fully realized until later conflicts like World War II.
For instance, the Schlieffen Plan predicated on Germany's need to fight a two-front war, did not primarily bank on the use of tanks or other armored vehicles. Likewise, during the Battle of Verdun, it was a war of attrition involving enormous toll of human life, not a battlefield dominated by the tactical use of armored vehicles.
A careful analysis of the different strategies used during the war, such as those in the Battle of the Marne and the invasion of Belgium, underlines the focus on infantry and artillery, not on the decisive implementation of armored warfare. This reflects the fact that the strategic potency of armored warfare was a lesson yet to be learned by World War I's conclusion.
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