It is July 2, 1863, and the afternoon temperature in Gettysburg has reached the low 80s.

Daniel Sickles, the commander of a Federal force of ten thousand men, has set up his field headquarters on the outskirts of town beneath a large swamp white oak on Peter Trostle’s farm.

The shade of the oak tree, then seventy-five years old and of good size, offered a welcome respite from the damp heat wafting across the open farm fields.

Rain was coming.

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