The lessons of D-Day and World War II ring true 75 years later as the Army marks its 244th birthday and prepares for a deadlier, more complex future battlefield, Army Chief of Staff Gen. Mark A. Milley said.

Speaking during an Army birthday celebration at the Pentagon—the service’s birthday is June 14—Milley talked about the deadly price paid by the Army during the Normandy campaign.

Nineteen U.S. Army divisions participated in the campaign, six of them on D-Day, Milley said. During that six- to seven-week campaign, 37,000 American soldiers were killed in action.

At the same time in the Pacific, the Army suffered 16,000 killed in the Central Pacific and another 9,000 killed in the South Pacific, he said.

The term “great-power competition” often gets thrown around, but “we should always pause for what that really means,” Milley said. “What that really means is significant challenges, significant casualties, significant battles.”

In a recent conversation with a veteran of the Battle of Carentan, which was part of the Normandy campaign, Milley said he asked the 96-year-old, two-time Silver Star recipient what lessons he drew from his combat experiences.

“He looked at me and he said, ‘General, never let it happen again,’ and he’s so right,” Milley said.

To do that, the Army must focus on readiness, modernization, managing talent and “by having a large, capable military force,” he said.

“Peace through strength is not a bumper sticker,” Milley said. “It’s a very, very true thing that keeps the peace in the world.”