The practice of two or more sovereign states uniting for security and external affairs is called

The practice of two or more sovereign states uniting for security and external affairs is called confederalism. This means that the states come together to form a confederation where they retain their sovereignty, but delegate some powers to a central authority to handle certain matters such as defense and foreign affairs. It is different from federalism where the central government has more power and the states have less autonomy. Confederalism is a way for states to cooperate and work together while maintaining their independence.