军事 (jūn shì) refers to military affairs as a broad concept — strategy, operations, and the military as an institution. It contrasts with 军队 (jūn duì), which means the concrete 'armed forces / troops' as an organization, and with 武装 (wǔ zhuāng), which emphasizes 'armed forces' or 'weapons' as a physical or figurative force. 军事 is often used attributively (军事力量 'military power').