Who arrests Sam? What is he accused of?

Sam is arrested by two cattle thieves who frame him for being a cattle thief. The cattle thieves proceed to present Sam to General Putnam.
The story My Brother Sam is Dead gives a different perspective on the American Revolution. The story is centered on the Meeker family and demonstrates the conflicts that emerged at the family level with regards to the war. Different family members supported different sides of the conflict, leaving families torn apart. In the Meeker family, Sam supported the rebels, while the rest of his family sided with the British. Tim, Sam’s younger brother, does not fully support any side.
Tim’s father dies on a prisoners’ ship after he is kidnapped. Sam is executed after he is falsely accused of stealing cows. The irony of Sam’s situation is that the cows stolen by the cattle thieves belonged to his brother and he was trying to recover them. However, he is overpowered by the thieves and framed for the theft. Despite efforts by his family to clear his name Sam is still executed.

In chapter 12 of My Brother Sam Is Dead, Sam has returned home from the war. He tells Tim about the great hardships endured by soldiers on the front line, including hunger. Sam confesses that, when hungry himself, he stole some cattle to survive. To make sure that his family doesn't suffer the same fate, he advises Tim to slaughter their cattle and freeze the meat over the winter. Tragically, Tim doesn't heed his brother's sage advice, with serious consequences.
One night, some thieves descend upon the farm and steal some of the family's cattle. Sam immediately heads out after them but is overpowered by the thieves and brought back to the farm tied up and unable to escape his fate. The cattle thieves turn Sam over to General Puttnam, who's declared that anyone found guilty of stealing cattle will hang. Sam is entirely innocent, but despite the best efforts of his family, the powers that be are not remotely interested in justice. They need a convenient scapegoat, and Sam perfectly fits the bill. Yet Sam earns our admiration by his remarkable stoicism and courage in the face of his impending death and the way he remains strong for his family.