I've been interested for awhile why Germany became the primary instigator of the great wars of the twentieth century. Belloc (in "Europe and the Faith") seems to locates the blame with Prussia and the fact that Prussia was insufficiently evangelized given its geographical location (too north and too east).
What else could've caused the nationalism that ran amuck in Germany? It seems sentimentalism and religion of the heart rather than the head had a role; Germany's Pietism movement was a counter to those espousing a universal humanity. (In a way this isn't too surprising. Islam is a religion with more fervency than rationality and has proven itself very adept at hatred of outsiders.)
Michael Burleigh in Earthly Powers writes of this (although he says it isn't of itself explanatory; there were many other factors):
Seventeenth-century German Pietism influenced the Romantic cults of the self and of God being present in nature. Pietism meant a faith based on love of Christ rather than intellectual subscription to a creed...Pietism involved not just the individual's direct experience of God, but acknowledgement of God's presence in wider fellowships and communities. These included the family, church and nation, the units within which it was really possible for human beings to know each other. A sermon delivered in 1815 sentimentalized this intense feeling of belonging:When a man speaks of the fatherland, he includes in this idea everything he loves on earth: the bosom of his parents, his circle of brothers and sisters, the family altar, his childhood playgrounds, the dreams of his youth, the places of his education, his field of work, and those thousand bonds that link him with his fellow citizens, the same language, the same customs, the same nationality, the same common life, common names, common possessions, common renown, common welfare, common sorrows...Pietism contributed to a spiritual climate in which such collectives as the nation became vehicles of intensified worship...