The idea to form the league of nations came from President Woodrow Wilson
Actually the idea for forming the league of nations was originated by Sir Edward Grey while he was British Foreign Secretary.
Nationalism was a key factor in the Balkan crisis.
The conflict between Serbia and Austria -Hungary was founded on the need of Serbs of Austria-Hungary . Russia, the mother of all slavs, intervened for Serbia while Germany , being in an alliance with Austria-Hungary supported them. This caused war between the two alliances.
Ottoman territories came under the control of allied powers, but austria-hungary territories gained the the ability to establish their own governments.
Russian troops suffered huge casualties and eventually refused to continue fighting.