Pope Francis prayed for an end to all war during a mass at the Sicily-Rome American Cemetery, just south of the Italian capital, on Catholic All Souls' Day commemorating the dead.
Francis laid flowers on the graves of some of the 7,860 American soldiers buried there as well as another 3,095 missing in action while trying to liberate southern Italy and Rome during World War II.
The pope said the thousands of white headstones should stand as a call for peace, saying "no more war, no more of these useless massacres.''
The pontiff lamented that humanity has not learned, or not wanted to learn, the lessons of war.
"How often in history, when men think of making war, they are convinced of bringing a new world, they are convinced that they are making spring. It ends in winter; ugly, cruel, a reign of terror, of death,'' the pope said.
Francis said that remembering the many young people who died in World War II is even more important today, when "the world once more is at war and is preparing to go even more forcefully into war."
As he left the cemetery, Pope Francis wrote in the visitors' book: "This is the fruit of war: hate, death, vendetta. Forgive us Lord."