BOLOGNA, Italy—For more than seven decades, Martin Adler treasured a black-and-white photo of himself as a young American soldier with a broad smile with three impeccably dressed Italian children he is credited with saving as the Nazis retreated northward in 1944. On Monday, the 97-year-old World War II veteran met the three siblings—now octogenarians themselves—in person for the first time since the war. Adler held out his hand to grasp those of Bruno, Mafalda and Giuliana Naldi for the joyful reunion at Bologna’s airport after a 20-hour journey from Boca Raton, Florida. Then, just as he did as a 20-year-old soldier in their village of Monterenzio, he handed out bars of American chocolate. “Look at my smile,” Adler said of the long-awaited in-person reunion, made possible by the reach of social media. It was a happy ending to a story that could easily have been a tragedy. The very first …