How did tensions in Europe lead to war?

First of all, the factors that created tensions in Europe are militarism, alliances, nationalism, imperialism, and assassination. Here's why:

1. Militarism, Europe had a large, strong standing army which lead to patriotism but also fear to some people.

2. Alliances, such as the "triple alliance" made up by Germany, Austria - Hungary, and Italy, which ironically, were made to prevent war in Europe but actually drew Europe into war.

3. Nationalism, because it caused intense competition among nations with each seeking to overpower the other.

4. Imperialism, in which a stronger country dominated and weaker country or territory.

5. The assassination of the Archduke of Austria - Hungary, Franz Ferdinand.