As described at mlb.com, Yankee Hall of Famer Yogi Berra was stationed aboard a naval vessel offshore during the Normandy landings, the operation code-named "Overlord." Other big league players who served in the armed forces during World War II include Ted Williams, Joe DiMaggio, Bob Feller, and Warren Spahn. Almost without exception they were modest, selfless, patriotic-minded citizens just doing their duty. The article linked above also refers to the letters exchanged between Baseball Commissioner Kenesaw Mountain Landis and President Franklin Roosevelt, who urged that baseball be continued in spite of the war, for the sake of national morale. With some of the best players serving in uniform, the quality of play fell noticeably, and the minor leagues almost evaporated. That was what led to the creation of the All American Girls Professional Baseball League, featured in the movie A League of Their Own.