Both Uses of

belligerent
in
The Magic Mountain

On that they were in fierce agreement—as worlds, as orders, as professions; and a child of peace found it worth his while to listen to Naphta talk about the martial monks of the Middle Ages, who, although ascetics to the point of exhaustion, were likewise filled with a spiritual lust for power and did not refrain from bloodshed in order to bring about the City of God and its transcendent world dominion; or about belligerent Knights Templar, who considered death in battle against unbelievers more meritorious than death in one's bed and for whom slaying and being slain for the sake of Christ was no crime, but the highest glory.†
Chpt 6.6 *

Asceticism was even his basis for reproaching the humanist whenever he trumpeted peace and happiness; Naphta would belligerently accuse him to his face of love of the flesh (amor carnalis) and love of physical comfort{commodorum corporis), call it utter bourgeois irreligion to ascribe the least value to life and health.†
Chpt 6.6

belligerently = with an attitude of one eager to fight

Definition:

hostile (the attitude of one eager to fight); or one already engaged in a fight or war