RICHMOND, Va. — The Civil War Trust has teamed up with the state to complete a $3.2 million campaign protecting 285 acres at Gaines’ Mill, where Gen. Robert E. Lee had his first major victory as commander of the Army of Northern Virginia.

The preservation greatly expands the number of protected acres at Gaines’ Mill, the bloodiest chapter in the Seven Days’ Battles, making it a “monumental achievement” in the trust’s history, president James Lighthizer said....