what was an effect of nationalism in europe in the early 1900s?

Well, the most important effect of nationalism in Europe in 1900s was the World Wars. Citizens of many countries were unhappy with the economy and the way things were in their states. This, along to the critical approach towards other nations and the growing national socialism led to the outbreak of military conflicts, pogroms, and eventually the holocaust and facism during the World War II. One important thing is that were talking here about very extreme nationalism.