Theory:

Then someone, I don’t know who, brought out a football. Greatcoats were dumped in piles to make goalposts, and the next thing we knew it was Tommy against Fritz out in the middle of no man’s land. Hans Wolf and I looked on and cheered, clapping our hands and stamping our feet, to keep out the cold as much as anything. There was a moment when I noticed our breaths mingling in the air between us. He saw it too and smiled. “Jim Macpherson,” he said after a while, “I think this is how we should resolve this war. A football match. No one dies in a football match. No children are orphaned. No wives become widows.”

“I’d prefer cricket,” I told him. “Then we Tommies could be sure of winning, probably.” We laughed at that, and together we watched the game. Sad to say, Connie, Fritz won, two goals to one. But as Hans Wolf generously said, our goal was wider than theirs, so it wasn’t quite fair.
Explanation:
 
As Wolf and Jim finished talking, Jim saw someone brought a football. Their long overcoats were carelessly piled up to make goalposts. It was a friendly match between the Tommies and Fritz, in the middle of no man's land. Jim and Wolf were cheering, clapping for their teams and stamping their feet, as it was cold. There was a brief moment when Jim was able to feel how he connected with Wolf - their breaths mingled in the air. They both were inhaling the same air, even though they were enemies. Wolf felt the same and smiled. He told Jim that the thought that football matches should be how they should resolve that war. Nobody dies in a football match, and no child loses his father, no wife loses her husband.
 
Jim replied that he would prefer cricket, then he can be sure of winning. This refers to the general fact that Germany is famous for its football skills and England is the pioneer of the cricket. Then they laughed together and watched the game. Unfortunately for Jim, Fritz team won. Wolf's team had scored two goals, while Jim's team scored one. However, Wolf was generous enough to say that their goalpost was wider, and the game wasn't fair.
  
Reference:
National Council of Educational Research and Training (2008). Honeydew. The best Christmas present in the world - Michael Morpurgo (pp. 9-16). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.