《宋书·列传·卷五十》
官军登岸之后，都拼死奋战，司马休之的部队挡不住，只好退却，官兵乘机追击，司马氏部队瞬间逃散。
既得登岸，殊死战，贼不能当，引退。因而乘之，一时奔散。
After the imperial army landed, they all fought desperately. Sima Xiuzhi's troops could not hold them back, so they had to retreat. The imperial army took the opportunity to pursue, and Sima's troops fled instantly.
<s>[INST] Kindly elucidate the central themes in this passage in English: 既得登岸，殊死战，贼不能当，引退。因而乘之，一时奔散。 [/INST] After the imperial army landed, they all fought desperately. Sima Xiuzhi's troops could not hold them back, so they had to retreat. The imperial army took the opportunity to pursue, and Sima's troops fled instantly.</s>
Kindly elucidate the central themes in this passage in English: 既得登岸，殊死战，贼不能当，引退。因而乘之，一时奔散。
user
After the imperial army landed, they all fought desperately. Sima Xiuzhi's troops could not hold them back, so they had to retreat. The imperial army took the opportunity to pursue, and Sima's troops fled instantly.
assistant