Chicago Folks Operetta has carved a niche over the past decade by specializing in long-forgotten hits from the “Silver Age” of European operetta in Vienna and Berlin in the 1910s and ’20s—schmaltzy, tuneful romantic comedies that evoke nostalgia for a simpler age before the First World War shattered the established order of European imperial politics and culture. But now—marking the 100th anniversary of America’s entry into World War I—CFO has taken a bold gamble on the long-overdue Chicago premiere of Kurt Weill and Paul Green’s 1936 operetta Johnny Johnson. Though not a success in its time, Johnny Johnson is a work of landmark historical significance. And despite some flaws, this production packs a real emotional impact with its quirky yet lyrical music, expressionistic visual design, and timeless theme of a common man caught up in the madness of war…