During the first great war(WWI) countries had formed defensive alliances with one another. So if 5 countries were in an alliance together, and 1 country was offended by someone, instead of 2 countries going to war, all 5 countries plus who ever started the conflict would all go to war. In case of WWI, instead of 2 countries going to war after archduke Franz Ferdinand was assassinated, countries from Europe and Asia went to war.