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Movies that deal with historical events--as "Valkyrie" does--are best made in semidocumentary style. They should be comprehensive, and they should adhere to the historical record. Consider "The Longest Day" (1962) about D-Day. It was based on Cornelius Ryan's book. "A Night to Remember" (1958), about TITANIC's sinking, was based on Walter Lord's book of the same name.
"Sink the BISMARCK!" (1960)? A good movie, if loose with the facts on occasion. The German fleet commander, Admiral Gunther Lutjens, is portrayed as a rabid bad guy (the opposite of the Admiral's actual character and personality). BISMARCK is shown taking torpedo hits from Captain Vian's destroyers (she didn't), sinking a destroyer (she didn't), and shooting down Swordfish torpedo bombers (she didn't).
My point? If you're making a movie dealing with an historical event, stick to the historical record, to the known facts. The events speak for themselves.