If history has shown anything, it has underlined both the importance and difficulty of preparing for the unexpected. A trained and ready Army must possess a sound doctrine, competent leaders, and effective, rugged equipment. Just as important to success is the Armys capacity to change. It must be able to rapidly adapt existing organizations, tactics, techniques, and procedures to meet the demands of emerging situations. How our military leaders did just that in the past is the subject of this focused essay. World War I - "The Great War"- was no less of a contingency operation than the many smaller overseas missions that the U.S. Army has undertaken over the past decade. While the general nature of that earlier conflict was well known to the U.S. Armys leaders prior to the deployment of the American Expeditionary Forces (AEF) to Europe in 1917, many of the specifics involved with raising a force that could fight effectively in the harsh trench warfare environment of that period were not. In fact, the small size of Americas prewar Army and the desperate need of its European allies for fighting forces meant that large numbers of U.S. Army troops entered combat with minimal preparation for the task at hand. The ability of American units and their commanders to identify problems and correct them in a systematic fashion thus became critical to the AEFs growing effectiveness and ultimate success on the battlefield. As we commemorate the eightieth anniversary of this nations involvement in World War I, it is entirely appropriate to recall our earlier experience to determine what might be relevant today. The "intellectual fieldcraft" that served the AEF so well during World War I remains a vital part of our heritage, one that ultimately led to the establishment of the Center for Army Lessons Learned (CALL). Similarly, the Armys postwar attempt to generalize from that earlier experience--always a more difficult chore--also contains lessons for those seeking answers to