Today, people from around the world are gathering in Normandy, France, to mark the 70th anniversary of the D-Day invasion. While World War II was fought on numerous battlefields and beaches in Europe and the South Pacific, it was won on the assembly lines of America’s factories.

Unfortunately, too many people today fail to realize that.

I have always had an interest in World War II era manufacturing. Despite all the numerous movies showcasing various aspects of the war, I don’t think there’s ever been one focused on the vital behind-the-scenes role that assembly lines played.

Thousands of unsung guys and gals toiled around the clock in countless factories that turned out all the engines, guns, tanks, trucks, ships and planes that made the Allied victory possible. They truly were war heroes, but they never received the kind of appreciation they deserve. I also have utmost respect for some other unsung folks who played a key behind-the-scenes role—all the merchant mariners who delivered everything to the ports and the front lines.

I recently came across a terrific new book entitled Images from the Arsenal of Democracy (Wayne State University Press). It’s filled with more than 300 pages of photos showing a wide variety of assembly lines in Detroit and elsewhere that mass-produced millions of parts, subassemblies and finished products 70 years ago. It showcases why the four-year-long war effort was the assembly line’s finest hour.

There are lots of great photos in the book showing people riveting, operating machine tools and assembling wire harnesses. One of the best images shows a long line of B-29 cockpits moving down the line at a Chrysler plant. That was back in the day before anyone ever heard of ergonomics, lean manufacturing, robotics, battery-powered tools, CAD/CAM, paperless work instructions, 3D printing and other things that we take for granted today.

But, what they accomplished is simply amazing. In fact, the statistics are staggering. American assembly lines cranked out more than $186 billion of equipment and supplies that made the Allied victory possible.

There’s never been anything like assembly’s finest hour. Hopefully, there never will be again.