Why were tensions high in europe in early 1900's?

They were high because of the little issues and things such as the growth of nationalism,  imperial rivalry, and the arms race.

The league of nations was found in 1919 to keep peace after world war l.but it failed to prevent world war ll in 1939,so in 1945 the nations of the world set up a new organization,the united state (un).since then, the un has encouraged international cooperation and worked to resolve conflict