Since they failed to resolve key issues and created resentments between countries, twentieth-century conflicts such as World War 1 and World War 2 were often followed by periods of even more fighting
A lot of issues and concerns were opened up during the World War 1 and World War 2, thus, by the end of these wars, some of these issues were still unresolved, which in turn bred hatred and became the cause of follow-up wars.
