这是最基本的概念，作定语是形容词：

We should not focus on how to fight each other but how to prevent military action.（形容词）

They all do military training as well.（形容词）

作主语、宾语、表语是名词，且有冠词或复数：

The military were plotting a coup. （名词）
该军方当时正在密谋政变。
《柯林斯英汉双解大词典》
If both militaries become locked in competition, these opportunities disappear. （名词）
如果两国军事陷入竞争，这些机会也就会消失。