Answer:
To form an army for defense
Explanation:
A confederation is a unification of nations or states that have come together to be united as a whole with a main goal in common, they usually do not want to loose power over their jurisdiction so the main reason to create a central authority is to rule them all and serve as a referee in interstate disputes, as well as to have a main representative to the whole world who is able to raise an army and defend the country.