Characterized by some authors as a rehearsal for the First World War, the Russo- Japanese War was arguably the world's first modern war. During this war, the lethality of weapons on the 20th Century battlefield was clearly demonstrated. Recording the events of the Russo-Japanese War were military and civilian observers from every major power of the time. These observers wrote voluminous accounts of the war that clearly illustrated this new battlefield destructiveness. The research question of this thesis is what tactical lessons were available to the observer nations of the Russo-Japanese War that were not used in their preparations for World War I. This paper will look at both observer accounts of the war and professional journal articles written soon after the war to consider this question. To answer this question, the stationery Siege of Port Arthur and the maneuver Battle of Mukden are used as representative battles of this war. Reports from these two battles clearly demonstrate the lethality of modern warfare and foreshadow the combined effects of hand grenades mortars, machineguns, and field artillery in World War I.