What was America feeling at the dawn of the 20th century? Protean songwriter-performer-director-producer George M. Cohan had his finger on the pulse of this so-called American Century. Immortalized on film by James Cagney, the legendary Cohan emblazoned his signature with hundreds of songs including “You’re a Grand Old Flag,” “Over There” and “Yankee Doodle Boy.” His story cracks open a crucial time capsule.