Why do countries ally themselves militarily with others

Many countries ally themselves militarily with others to facilitate world peace. Otto von Bismarck of Germany founded this proponent. Bismarck believed that the European nations could ally in various ways in order to maintain a peaceful balance of power. The alliances emerged were quickly stable onward resulting to  unification and expansion of Germany that took place throughout early 1870.

social contract theory, nearly as old as philosophy itself, is the view that persons' moral and/or political obligations are dependent upon a contract or agreement among them to form the society in which they liv