根据以下内容，回答下列各题。

The large part which war played in English affairs in the Middle-Ages, the fact that the control of the army and navy was in the hands of those that spoke French, and the circumstances that much of English fighting was done in France all resulted in the introduction into English of a number of French military terms．The art of war has undergone such changes since the battles of Hastings, Lewes, and Agincourt that many words once common are now only in historical use．Their places have been taken by later borrowings, often like wise from French, many of them being words acquired by the French in the course of their wars in Italy during the sixteenth century．Yet we still use French words of the Middle Ages when we speak of the army and the navy, of peace, enemy, battle, soldier, guard and spy, and we have kept the names of officers such as captain and sergeant．Some of the French terms were introduced into English because they were needed to express a new object or a new idea．In other cases a French and a native English word for the same thing existed side by side．Sometimes one or the other has since been lost from the language; but sometimes both the borrowed and the native word have been still in common use．

The main idea of this passage is that ______．