YOKOHAMA, Japan—Japan’s players sprinted onto the mound and the collection of All-Stars hoisted their manager up and down as if on a trampoline. They beamed when hanging shiny gold medals around each other’s necks, fulfilling a national mission with the first Olympic baseball title in their history. American players didn’t seem overly upset. The released veterans, prospects, and career minor leaguers thought they had given their best. “I really feel like we left it all out there,” pitcher Nick Martinez said after Japan eeked out enough runs to win Saturday night’s gold-medal game 2–0. Munetaka Murakami, at 21 the youngest player in Japan’s starting lineup, hit an opposite-field homer over the 16-foot wall in left-center on a 2–2 pitch from Martinez (1–1) in the third. Martinez winced as the ball landed in the fourth row of the empty blue seats. “I thought we had him set up nice for a …