This form of government is defined in Wikipedia¹ as:

“A confederation (also known as a confederacy or league) is a union of sovereign states, united for purposes of common action often in relation to other states. Usually created by a treaty, confederations of states tend to be established for dealing with critical issues, such as defense, foreign relations, internal trade or currency, with the general government being required to provide support for all its members. Confederation represents a main form of inter-governmental-ism, this being defined as ‘any form of interaction between states which takes place on the basis of sovereign independence or government. Confederation is almost as a Federation with the Federal Government being as a combination or alliance of all the states.”

The most notable example today is the European Union.

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1. Wikipedia is NOT my go to source but for something this basic, it’s the quickest. I will NOT make a habit of this on the site.