A more thorough explanation:

Definition: Casus belli is a Latin term that refers to an act or situation that provokes or justifies war.

• When Germany invaded Poland in 1939, it was seen as a casus belli for the start of World War II.
• The terrorist attacks on September 11, 2001, were considered a casus belli for the United States to declare war on terrorism.

These examples illustrate how a specific event or circumstance can be seen as a justification for going to war. In both cases, the actions of one country or group were seen as a threat to the safety and security of another, leading to a military response.