自漢高祖劉邦親率大軍主動出擊匈奴，在平城被圍脫困後；長期以來漢代的國防政策即以防備匈奴為主，且在「和親」、「納奉」之下勉強維持邊境的安寧，然經數十年的安定，及國力逐步厚植下，有識之士早已深知國家的對外政策絕不應建立在所謂的「和親」、「納奉」之下，希冀漢廷有所作為而改變政策；但因當時漢廷內部仍有諸多問題亟待解決，故無法對外發動戰爭；至武帝一朝歷經文、景兩帝的經營，終使漢代國力達到高峰，故有能力對整體國家戰略，作一宏觀性的重大轉變。

The national defense policy of Han Empire had focused on resisting the invasion of Xiongnu since Emperor Gaozu’s escape from the envelopment in the Battle of Baideng. Emperor Gaozu initiated the policy of heqin, which involved sending noble ladies to marry the Xiongnu leaders and paying annual tribute to the Xiongnu in exchange for peace between the Han Empire and the Xiongnu. Although the policy of heqin was debated and even censured by some ministers of great vision, poor economy held back Han Empire’s active military strategies until the Rule of Wen and Jing. The Rule of Wen and Jing enabled Emperor Wu to maintain a powerful army and employ an aggressive military strategy, which in turn dramatically expanded the empire and ultimately pushed the Han dynasty to its zenith.