“斡腹”用现代军事术语讲就是长途奔袭迂回攻击，绕到敌人正面防线侧后，攻击其防御薄弱地带的战术。“斡腹”的灵感很可能来自蒙古人的狩猎经验。在狩猎过程中，要通过四面合围狩捕猎物、捕杀猛兽，往往攻击其柔软的腹部更容易得手。使用“斡腹”战术的前提条件是有一支高机动、不依赖后勤的部队。
宋蒙战争开始后，南宋凭借长江天险，与蒙古形成了对峙局面，使蒙古数次南征均无功而返。1253年八月，忽必烈率抄合、也只烈、哈单、朵欢、脱脱、兀良合台等部蒙古骑兵，以及汉将董文用、董文忠、郑鼎所部共20万大军，从临洮出发，至白龙江支流包座河旁的塔拉分兵三路，开始了前所未有的历史大进军。
Climate: the Mongols and their horses were highly adapted to the dry and cold climate of the steppe and Siberian forests. Heat was tolerable for them， but humidity and wetness were far more troublesome. When the Mongols invaded southern Song，they suffered severely from diseases like dysentery， cholera， and malaria， as well as simply from the heat. Even the Northern Chinese had significant health problems fighting in the South under various Chinese dynasties.
金兀术、完颜亮叔侄俩的渡江之战，虽有一时得手，但最终都以失败告终。同样是面对这条大江，蒙古人却成功了。蒙古人的成功和南宋叛将刘整有关。刘整骁勇善战，很有军事才能，但为同僚所妒，又被贾似道排挤，被迫投降蒙古。
