The Civil War battlefield is a project that was worked on by Frank Heffley in books two to three, although he is seen working on it in the Diary Of A Wimpy Kid (film). During the Rodrick Rules, Frank is strict with Rowley because he is afraid Rowley might accidentally wreck it. In The Last Straw, after Frank threw away Tingy (Manny's favorite blanket), Manny mercilessly destroys the battlefield, and it's not even mentioned in the Dog Days. It later appears in Hard Luck when Greg shows the places where Rodrick put the flowers in his science project What type of music do flowers like best?.
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Frank working on his battlefield.
Uncle Joe teases Frank, as he said they are toys