Game concept
War Leaders
The game also links it as War Leader.
From the game's encyclopedia
The primary attacker and the primary defender of a War. Usually this is the character who instigated the war, and the Top Liege of the character who holds the War Target.
There are a variety of Casus Belli for many different kinds of war, some with different characters as the War Leaders.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
War Leaders is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The primary attacker and the primary defender of a War." The game also links it as War Leader. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.