Korean War parallels
There are striking parallels with the Korean war, which have no doubt escaped the limited intelligence and bad general knowledge of American audiences.
In August 1950 the invading Communists had almost overrun the country. The North Koreans had pushed South Korean and UN forces into what became the Pusan Perimeter. It was from this successful last stand fight that Korea was eventually liberated.
In the movie another evil overruns the country, leaving only Pusan free.