Customary International Law (CIL) is a big part of international law. It comes from the regular actions of countries along with their belief that these actions are required by law. This idea is called "opinio juris." But can countries really change or challenge these customs? Yes, they can! Let’s break it down.

1. What is State Practice and Opinio Juris?

• State Practice: This includes the actions that countries usually take when it comes to a legal rule. This can be anything like agreements between countries, how they act in military situations, or how they interact with each other.
• Opinio Juris: This is about the belief that countries are acting with a sense of legal responsibility.

2. How Can Countries Challenge Customary Norms?

• Countries can change the existing customs just by acting differently. For example, if many countries start to ignore a certain accepted practice—like a way to follow rules at sea—then that practice might slowly stop being the norm.
• A good example of this is the changing rules about using force. What was once accepted can be challenged, especially as the world changes.

3. Creating New Customary Laws

• On the flip side, new ways of doing things can create new customs. If a country starts doing something new and many others accept it, a new rule can form if there is also a belief that it should be considered law.

4. The Role of Treaties

• Treaties, or formal agreements between countries, can also change customary international law. When a lot of countries agree to a treaty that goes against an existing custom, it can spark a change, especially if it shows a shift in how countries are acting.

To sum it up, the actions of countries are very important in both challenging and creating customary international law. The world of international law is always changing because of the interests and actions of countries, making it a fascinating area to study. The rules can shift and evolve based on how countries behave and what they believe.