2)The rise of aggressive nationalism among some of the nations defeated in World War I
Germany was defeated in WWI, this lead to feeling of necessary nationalism which its governors took advantage of, in the time another nations with a strong nationalism feeling helped them initiate the war.
3)The rise of communism in Soviet Russia, which was seen as a threat by many europeans nations.
Soviet Russia was winning a lot of force while under the communist regimen, while this was at a high social cost given the low quality of life they had, the state got wealthier and started to make big investments in army and development. This was seen as a threat by the european nations that had already saw how Russia started anexing countries to its union.