Complex sentence: The 1st Battalion, 81st Infantry Regiment was assigned to defend Cape Merkus but did not depart Rabaul until December, as it needed to be reorganised after suffering casualties when the ship it was travelling on from China was sunk.
Split into simpler sentences: The 1st Battalion, 81st Infantry Regiment was assigned to defend Cape Merkus. However, it did not depart Rabaul until December as it needed to be reorganised after suffering casualties when the ship transporting it from China was sunk.