Operation POSTERN was a joint forcible entry operation in New Guinea, 1943, spearheaded by land forces from the Australian I Corps. It was a truly joint and combined operation that involved all three services and forces from Australia and the United States. A retrospective glance at POSTERN affords valuable insights for future maritime campaigning. For the foreseeable future, the core military problem for amphibious and littoral operational planners is likely to remain gaining access to maritime terrain across multiple contested domains, and generating sufficient combat power ashore to stage at or seize advance bases. Operation POSTERN offers important operational lessons in theater-shaping, deception, manoeuvre, and advance basing.