Answer: First World War put an end to 19th century (sometimes 19th century is called "long century", i.e. 1789-1914 whereas the 20th century a "short" one, i.e. 1914-1989, or -2011 ?), great empires ended (ottoman empire, austro-hungarian, German Second Empire and in 1917 Russian Romanov empire). In arts but also in philosophy there was less attachment to older 19th-century perspectives. Avant-garde in arts search for the way how to cut off the weight of traditions. In philosophy and literature one can find the same ....but these trends are not always optimistic (existentialism, phenomenology), liberal welfare-society is constantly questioned, belief in power of unconscious (as a substitute for god) as well. Europe had an headache from the fast secularization....fast changes that occurred in 19th-century Europe.
It was necessary to win the war because the south wanted to separate and that would make the south and north both weak also the south wanted slave but the north didnt
