Nationalism was a huge cause of World War 1. Nationalism meant that a country was very proud and wanted to be the best which leads to greed, false hopes, and aggression. Militarism was also a big part because countries were part of an arms race to have the biggest and strongest navy and military.
For example, Great Britain had the biggest and most powerful navy but Germany had the intention to build a bigger navy which Great Britain felt very threatened by. This all built tensions between countries and helped spark World War 1.