europe and asia are divided by the ural mountains so the answer is
the mongols were nomads who started out in northern china and raided their neighbors, the chinese all the time, thats why the great wall was built. . they kept on moving west into europe.
while there was much buildup pre-world war i, it is commonly understood that the assassination of archduke franz ferdinand on 28 june 1914 in sarajevo pushed the world to war as a result of a complex web of treaties and diplomatic obligations that quickly were triggered and resulted in sides being chosen.