WASHINGTON -- Seventy-five years ago today, about 70,000 Marines and sailors, backed up by about 400 ships, made the assault landings on Iwo Jima, a Pacific island that is only several square miles in size.It was the bloodiest campaign in American history per square mile, the chairman of the Joint Chiefs of Staff said."They slugged it out, inch by inch, yard by yard for the next 36 days," Army Gen. Mark A. Milley said, resulting in almost 50% of the battle's participants being killed or wounded.Of the 22,000 Japanese defenders, only about 1,000 survived the battle, he added.Milley noted that his father, Alexander Milley, was a Marine who fought in Iwo Jima and other campaigns during the war.The chairman was the keynote speaker at a commemoration marking the 75th anniversary of the start of the battle of Iwo Jima at the National World War II Memorial in Washington.Last year, Milley visited Iwo Jima with his counterpart from the Japan Self-Defense Forces. "It's an incredibly humbling experience to walk through that island … trying to imagine what hell on earth was like, and that was Iwo Jima," he said.From 1914, the start of World War I, to 1945, the end of World War II, the world was engaged in two of the biggest wars that have ever been fought in the history of humanity, Milley said.But those who fought in the wars fought for something -- they fought for a better peace, he noted. At the end of 1945, he said, that greatest generation bequeathed a set of freedoms and a world order that has held good since then.This is the 75th year of no great-power war, and that is what World War II veterans, including those at today's event, fought for, the chairman said.Today, the United States is in a great-power competition with China and Russia. "It is incumbent upon all of us to make sure that great-power competition of today stays at great-power competition and great-power peace and doesn't yet again turn into great-power war," he said."Let us resolve once again to never let it happen again," Milley said. "Let us resolve that we're not going to have another great-power war, because the slaughter that is involved is beyond the imagination. It's beyond the pale."Everyone who wears the uniform is committed to maintaining the great-power peace, he concluded.