Its a metaphor
The movie is best viewed as a metaphor for WWII. Look at halftime and the players; Germans ahead, American beat up, Russians starving, British pulling the load. In the real world, Germany was at its peak worldwide in June 1942, the halfway point of the war, the US was rocked after Pearl and the rapid expansion throughout the Pacific, Midway was just barely won at terrible cost, Britain had won El Alamein, Russia was barely hanging on in the East and losing hundreds of thousands but holding on, French resistance had started up. Then the end, its up the American (everyone counted on the US to carry a lot of the load) and he comes through. Its a parallel, clear and simple. Watch the beginning, wild German victories in the game and in the war.
The message is: never give up, nothing is written. Nothing. We have freedom of choice, life is not a script. Just cause you are losing it does not mean that you can not win. That, I think, is the message Huston wanted to convey. Life is not a movie script, determination and guts count!