Final answer:
European countries formed alliances for political rivalries, territorial conflicts, and mutual protection. The North Atlantic Treaty Organization (NATO) was formed in response to the threat of Soviet aggression. Historical grievances and unresolved conflicts also played a role in alliance formation.
Step-by-step explanation:
European countries formed many alliances due to political rivalries, territorial conflicts, and the need for mutual protection. These alliances helped countries secure their interests and maintain a balance of power. For example, the formation of the North Atlantic Treaty Organization (NATO) in 1949 was a response to the perceived threat of Soviet aggression. Countries in Western Europe joined together to provide collective security and deter potential invasions. Additionally, historical grievances and unresolved conflicts further contributed to the formation of alliances in Europe.