The French-speaking Normans eventually defeated Old English-speaking Saxons at the Battle of Hastings on October 14, 1066 — which had a larger and more pronounced effect on the development of the English language than any other event in history. Within the course of a few centuries, English went from being a strictly Germanic language to one infused with a large Latinate vocabulary, which came via French.
The English adapted from their French-speaking Norman invaders many words surrounding elements of war. We send to far away lands, for example, our soldiers, a word that came to us in Middle English from the Old French word soldier from soulde, the Latin word for a "gold coin of the Roman Empire."
Our soldiers are sent off in battalions to do battle, from Old French bataille, based on late Latin battualia describing "military or gladiatorial exercises" — from the Latin root verb "to beat."
In the course of the war, sometimes a sergeant (Old French, sergent) or commander calls for a siege, based on the Old French sege — from asegier, a verb that means "to besiege." Originally, in Middle English, besiege meant to "sit down in front of."
The Old French verb armer means "to supply with weapons" and is the basis of our army, as well as armor and armory. Navy also came into Middle English from Old French, from the Latin word for ship, navis, which also forms the root of navigation.
A traitor is a person guilty of treason, both from Anglo-Norman French treisoun, meaning "handing over."
The word war itself is distinctly Anglo-Norman. The late Old English word werre, which evolved to modern English war, is from an Anglo-Norman French variant of the Old (and Modern) French word guerre.
Werre (Old English war) also shares a Germanic base with the word "worse." The old Germanic werra indicated "confusion, discord." (Modern German developed a different word entirely for war, krieg.) Middle English warrior is from Old Northern French werreior, a variation of guerreior, "to make war."
A war can end after defeat or retreat (Old French retraiter, "to pull back") or after a treaty (Old French traite), and this might lead to everlasting peace, which came to English from Latin pax via the French word pais.