Declaration of War
A declaration of war (also referred to as DoW or Dowing) is the diplomatic message sent to a nation to officially start a war. The sender of the declaration will be regarded as the aggressor and will often incur a stability loss.
In Heir to the Throne and Divine Wind, you can select one of a list of available Casus Belli, each fitting different war goals. Declaring war without a Casus Belli, or without selecting one from the list (the default) will always cause a stability hit of at least 2.