13 black-and-white wartime photos juxtaposed with their modern-day locations.

In Wiesbaden, Germany, history buff Peter Perry saw the perfect opportunity to take some photos that were a "little bit more creative or unique than your average tourist pic."

While visiting his dad at a military base in Germany, Perry realized that some of his favorite historical sites were just a short drive away. It was days after the Pulse nightclub shooting in Orlando, and the Wiesbaden city hall was displaying a rainbow pride flag on its balcony in a show of solidarity.

Perry remembered seeing an old photograph of Adolf Hitler on that same balcony, so he ran to a nearby drugstore and printed it out, "along with a bunch of other personal stuff," says Perry over the phone. "I didn't want to be that American in Germany only printing out a photo of Hitler."

Perry began a photo series that he calls "Then/Now," and it all started with this remarkable image:

He held up the printed out photo of Hitler, lined up the balcony and windows with the real-life city hall building and snapped away.

He posted the photo online, and — as things on the internet often do — the captivating photo soon made its way to Reddit, where it received thousands of upvotes.

That was only the beginning. Perry has since traveled all around Germany and Prague taking similar composites that combine historic photos with their modern-day locations.

Like this image of French occupation forces in front of the Wiesbaden city hall in September 1919:

And this image, showing a French military concert in front of the same building in 1919:

This picture shows British forces in 1925 standing in formation before a tree that still exists today:

Here he matched a photo of American forces — The 270th Engineer Combat Battalion, to be exact — on a victory march through German streets in 1945:

Some of the images reveal forgotten pop culture moments overseas, like this photo of The Doors performing an outdoor concert in Frankfurt in 1968:

And Elvis Presley's visit to Bad Nauheim in 1959:

In 1968, Warsaw Pact nations invaded Czechoslovakia, and the images of foreign tanks rolling down Czech streets are unforgettable:

Perry says he's fascinated by the wartime history of Europe because it's unlike anything you can find in America.

"In Boston and stuff, you have a lot of history," Perry says. "But fortunately, we’ve never had foreign tanks rolling on our own streets. As drastic and terrible as some of these wars are, it’s interesting and kind of cool of be able to see some of the stuff that America's own soil has, fortunately, never gotten the chance to see."

And this striking photo was taken in front of the Prague National Museum in 1968:

This image shows Oskar Schindler (whose story was told in "Schindler's List"), with the people he saved, outside of his enamelware factory in 1944:

Here, The British Army of the Rhine Scottish Guard stands in formation in 1927:

The French Guard in Wiesbaden in 1922:

If there's a theme to Perry's pictures, it's that looking back at history reveals just how far we've come.

Progress — whether it's a pride flag waving where a dictator once stood or a peaceful, bustling street that was once torn up by foreign tank treads — tends to reveal itself when you look back at the ghosts of the past.

History is full of horrors, but it's also full of lessons. Perry's "Then/Now" juxtaposition is a visually captivating way to bring those lessons to the surface.

"We still have a long way to go," says Perry. "But it shows how far we’ve come."