A more thorough explanation:

Definition: Casus foederis is a Latin term that means "the case of the treaty" or "the case of the agreement." It has three meanings:

1. International law: It refers to a provocative act by one nation towards another, which entitles the latter to call upon an ally to fulfill the terms of an alliance.
2. Treaty of alliance: It is a clause within a treaty of alliance that specifies such provocative acts.
3. Contracts: It refers to a case or an event falling within the terms of a contract.

Examples:

• Example 1: If Country A attacks Country B, and Country B has a treaty of alliance with Country C, then Country B can invoke casus foederis to call upon Country C to fulfill the terms of the alliance.
• Example 2: A treaty of alliance between Country X and Country Y may specify that if Country X is attacked by a third party, then it can invoke casus foederis to call upon Country Y to provide military assistance.
• Example 3: In a contract between a buyer and a seller, if the seller fails to deliver the goods on time, then it can be considered as a casus foederis, and the buyer can take legal action against the seller.

The examples illustrate how casus foederis can be used in different contexts. In international law, it is used to protect the interests of allied nations. In a treaty of alliance, it is used to specify the terms of the alliance and the actions that can trigger it. In contracts, it is used to define the events that can lead to legal action.