If there was one thing that caused both WW1 and WW2 what would it be?
One thing that caused both WWI and WWII would be intense nationalistic feelings. WWI was started when a Serbian nationalist shot the Austrian Archduke Francis Ferdinand in Sarajevo. Nicholas II of Russia felt as though he was the protector of all Slavs in Europe--this is why he backed Serbia in its war with Austria-Hungary. Germany felt as though it should have had more status in Europe so it built up its military which led to tension between Germany, Britain, and France.
WWII would not have been possible without WWI. Germany felt cheated by the Versailles Treaty and Hitler promised to restore the nation to past glory. He stated that the German people were ideal and that they should own Slavic land in the East. Italy felt as though their contributions for the Allied Powers in WWI were not appreciated enough so it backed Germany in WWII. Mussolini wanted to recreate the Roman Empire and soon began to claim land along the Mediterranean for Italy. Japan sought to drive out the European powers from Asia and create an Asia where Japan was the major power. This led to tension between Japan and the United States.