Why did countries form alliances? what was the purpose?

The purpose of an alliance is to (1) achieve joint strategic goals, (2) reduce risk while increasing rewards and/or, (3) leverage resources.

Explanation:

Definition of alliance: Coming together of two or more firms to create a unique organizational entity (such as a joint venture), in which each firm retains its individual identity and internal control. The purpose of an alliance is to (1) achieve joint strategic goals, (2) reduce risk while increasing rewards and/or, (3) leverage resources. Since an alliance is neither an acquisition nor a merger, it requires new control methods and new management skills.