World War I had important effects on the economy and society of France. We can see changes along three dimensions: social and economic structures, institutions and policies, and ideology and mentality. In this paper, we examine the changes along these dimensions, paying particular attention to state intervention in the economy and society, and focusing on economic modernization and the construction of the welfare state. First, we examine the economic and social characteristics of France before World War I, to understand the changes which occurred due to the Great War, as well as examine the immediate effects caused by the economic mobilization during the War. Next, we call attention to the various plans for economic modernization to overcome the relative economic backwardness, the change of attitude of the labor movement towards the state (the renouncement of "anti-etatisme"), the diminution of resistance to state intervention, and the development of "social reform". Third, we examine later consequences of the changes caused by the Great War. We argue that structural reforms in the economy and society after the World War II, i.e., the nationalization of industries, economic planning, and the social security system, were consequences of the changes that came about due to the Great War. One characteristic of postwar reform in France was that economic modernization was given priority over social reform.