ORIGINAL: bradfordkay
The situation in the screen shot seems to be an unusual one: at some point an allied unit marched along the Kokoda Track between Port Moresby and Buna (unknown to the casual observer as to which direction it took, but for the game effect this is of no consequnece) while the Japanese captured both Port Moresby and Buna by naval invasion. At no time has the Japanese player sent troops along the track to clear the route, because the game shows him that there are no enemy troops there.
Yes, I had some poor souls get forced out from PM when it was captured (first) and they had to march all the way to Buna. (Then when Buna got taken they ended up in Salamaua. ) I guess that is when it happened. Didn't seem like a good thing at the time but it did turn out to be usefull.