“German nation bonus earned after winning a multiplayer battle.”
Teutonic Fury is a specific battle tactic that only the Germans possess. After a multi-player victory it gives you a 15% attack boost towards your next attack. This is repeatable and you can keep this booster as long you keep winning, however once you lose a multi player battle you will loose the booster and will have to win another battle to regain it. This tactic carries over towards war as well.