The captain of the giant Royal Navy battleship called his officers together to give them a first morsel of one of World War II’s most closely guarded secrets: Prepare yourselves, he said, for “an extremely important task.” The secret was D-Day — the June 6, 1944, invasion of Nazi-occupied France with the world’s largest-ever sea, land and air armada. The diary writer was Lam Ping-yu — a Chinese officer who crossed the world with two dozen comrades-in-arms from China to train and serve with Allied forces in Europe.