A more thorough explanation:

A declaration of war is an official announcement made by a country's government stating that it is at war with another country. This declaration is usually made after diplomatic efforts to resolve conflicts have failed.

For example, in 1941, the United States declared war on Japan after the attack on Pearl Harbor. This declaration of war allowed the US to officially enter World War II.

Another example is when Germany declared war on France in 1914, which marked the beginning of World War I.

These examples illustrate how a declaration of war is a formal announcement that a country is entering into a state of war with another country. It is a serious decision that can have significant consequences for both countries involved.