Why are some militaries more effective in combat than others? In this dissertation, I argue that soldiers are effective in battle when they believe there is a close connection between their national identity and the goals of the war. Troops will have strong morale, few disciplinary incidents and take initiative on the battlefield when they identify the goals of the battle with the emotionally resonant myths and symbols of their own national identity. The existing literature on this topic offers three main arguments. The first claims that soldiers are more effective in battle when they have close emotional and psychological relationships with their fellow soldiers. Small-unit cohesion in the form of close, almost familial relationships between soldiers leads them to fight for one another, thus increasing effectiveness on the battlefield.  The second argument claims that soldiers who come from democracies are more effective in combat than non-democratic soldiers. These soldiers take more initiative on the battlefield, have more faith in their leaders, and are more committed to their country than soldiers from authoritarian states. Third, realists argue that nationalism leads to combat effectiveness, but define nationalism in terms of a threat to the territorial nation-state. Nationalism increases combat effectiveness when the territorial state is threatened, threatening the political survival of the nation.  I assess the strengths and weaknesses of each argument through two in-depth case studies comparing British, Indian and Australian troops in the British military during World War II: The battle for North Africa against the Germans and Italians, and the defense of Malaya against the Japanese. The evidence suggests that national identity theory is the most powerful explanation for combat effectiveness, though small unit cohesion also plays an important role.  This dissertation has important scholarly and policy implications. It helps policy makers assess their own military's potential combat effectiveness, as well as that of potential opponents. Theoretically, it contributes to our understanding of the role of nationalism in conflict processes and military power.