In a story like The Fighting Ground, I believe it is usually safe to say that the main problem or main conflict is the one between the great powers involved, in this case between the American revolutionary forces on the one side and the British and Hessians on the other. The characters in such a story are typically the "little people" who are caught up in the struggle and whose lives are changed by what happens to them. In The Fighting Ground, the main character is indeed "little." Jonathan is only thirteen years old. There are many examples of literary works in which the main problem or conflict involves two powers in an epic struggle, going all the way back to Homer's The Iliad. Leo Tolstoy's masterpiece War and Peace is another good example.
Jonathan's major conflict is deciding whom to trust, the American soldiers he is with or the Hessian soldiers he meets. Prior to meeting the Hessians, Jonathan has heard terrible stories about them, but once he meets them, he realizes they are good men that he can trust. Jonathan tries to save the Hessians in the end, but he's unable to do so. As Jonathan tries to come to terms with the terrible things he has seen that day, he realizes that men who fight wars are all humans and not necessarily evil. War forces humans to kill other humans, and this is the tragedy of war.
Jonathan's inability to remain loyal to the Patriots or the Hessians is in itself, representative of the novel's main conflict. This conflict lies within Jonathan, because he knows that if he chooses to side with the Patriots, he will be no better than the Corporal who shot that young boy's parents. On the contrary, if Jonathan chooses to side with the enemy of his people, he will not only be a traitor, but his fathers wound and service will also effectively be in vain. You can see how he walks along the metaphorical fence of the war, due to the fact that he ran back to the Minutemen camp after escaping the Hessians and decides to tell the Patriots of the hideout. He then cannot help but to lead the minutemen back to the house, but he then chooses to defy the Patriots by warning the Hessians that they are surrounded and their death is imminent. Jonathan then demonstrates that he is not loyal to them in this war either, because when they try to use him as a hostage, he slips away allowing them to be shot by the minutemen. The main conflict of this novel is surely internal, because once exposed to both sides of the external conflict (the war), Jonathan learns that the situation is not black and white, and therefore in this situation he cannot lay his own life down nor may he be directly responsible for the death of another. It is this realization, coupled with the feeling that he is being disloyal to his family and homeland, which tears a 13 year old Jonathan apart inside.
There isn't really a "main problem" in the book. Yes Jonathan doesn't know who to trust but I would say it is mostly about him in The Revolutionary War and him trying to escape back to his home. I mean he is only 13, my brothers that age and I can't image him in any war, could you image a 13 year old boy in a intense war like that?