Game concept
Offensive War Opinion
The game also links it as Offensive War.
From the game's encyclopedia
Being the main attacker in a war will gradually cause your Vassals to dislike you after 6 months. Civil Wars and Great Holy Wars are excluded from this. Once no longer in such a war, the penalty will slowly decay.
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.
Offensive War Opinion is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Being the main attacker in a war will gradually cause your Vassals to dislike you after 6 months." The game also links it as Offensive War. 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.