西晉人祖逖和劉琨，都是性格開朗、仗義好俠的志士。年輕時不但文章寫得好，而且都喜歡練武健身，決心報效祖國。當時，晉朝表面上還管轄著中原大地，但實際上已是內憂外患，風雨飄搖了。祖逖和劉琨一談起國家局勢，總是慷慨萬分，常常聊到深夜。
一天，祖逖又和劉琨談得十分興奮，劉琨不知什麼時候睡著了，祖逖卻久久沉浸在談話的興奮之中，不能入睡。“喔，喔，喔－－”荒原上的雄雞叫了起來，祖逖一躍而起，踢醒了劉琨：“聽，這雄雞啼鳴多么振奮人心呀，快起來練劍吧！”於是，兩人操起劍來，在高坡上對舞。從此，他倆每天清早聽到頭一聲鳴叫，一定來到荒原上抖擻神練起劍來。
劉琨被祖逖的愛國熱情深深感動，決心獻身於祖。一次他給家人的信中寫道：“在國家危難時刻，我經常‘枕戈待旦’（枕著兵器睡覺一直到天明），立志報國，常擔心落在祖逖後邊，不想他到底走到我的前頭了！……”
故事出自《晉書·祖逖傳》。“枕戈待旦”出自劉琨《與親故書》，形象地寫出了劉琨隨時準備殺敵報國的決心。後來用作成語，形容時刻警惕敵人，準備作戰。
lying with one's head pillowed on a spear,waiting for day to break
in the western jin dynasty there were two young men.one of them was zu ti and the other was liu kun.both of them were men of ideals and integrity who were chivalrous and of a sanguine disposition.they not only wrote excellent articles but also were fond of practising martial arts to keep fit, in order to render meritorious service to the country.both of them were chief clerks responsible for document administration in luoyang.although in appearance the jin dynasty had jurisdiction of the central plains comprising the middle and lower reaches of the haunch and threatened by foreign invasion.zu ti and liu kun often talked about the country's situation till late into the night,and each time they talking very excitedly again.liu kun fell asleep without knowing it,but zu ti was too excited to fall asleep." cock-a-doodle-doo,"came the crow of rooster in the wasteland.zu ti jumped up and kicked liu kun awake:"listen.how inspiring the rooster's crow is.let's get up and practised on a slope.from then on,they kept practising sword playing vigorously and energetically in the wasteland every day when they heard the first crow in the morning.deeply moved by zu ti's patriotic passion,liu kun was determined to devote himself to his homeland.once he wrote to his family:"at the time when the country is in dire peril,i am resolved to dedicate myselt to the service of my country.i often fear that i might lag behind zu ti in rendering service to the country,and,in fact,i am behind him..." the words" sleep with my head pillowed on a spear,waiting for the day to break"vividly described liu kun's determination to dedicate himself to the service of the country and to fight the enemy at any time.later,this set phrase is used to mean maintaining sharp vigilance and being ready to fight at any time."