Complex sentence: The Australian victory at Kaiapit enabled the Australian 7th Division to be flown in to the upper Markham Valley which accomplished the 7th Division's primary mission, for the Japanese could no longer threaten Lae or Nadzab, where a major airbase was being developed.
Split into simpler sentences: The Australian victory at Kaiapit enabled the Australian 7th Division to be flown in to the upper Markham Valley. This action accomplished the 7th Division's primary mission, for the Japanese could no longer threaten Lae or Nadzab, where a major airbase was being developed.