Belligerence is a natural disposition to be hostile or aggressive, or a warlike attitude. The belligerence of Roman gladiators made them excel in battle and instilled fear in their opponents.
Belligerence in people is characterized by a tendency to pick fights or be aggressive toward others. It can be something in your nature, if you're, say, a bully, or it can be an attitude you summon in the heat of battle, as when you're going up against a rival sports team. One thing's for sure, though, there is nothing peaceful about belligerence, which, when displayed by armies or countries, can lead to devastating wars.