Taken in 1918, this photo shows the near absolute destruction of Ypres, a crucial city in the fight for West Flanders.

The smaller image to the left shows the entirety of Belgium, which was a strategic byway between Germany and France. The larger map on the right is West Flanders, and Allied and German front lines ground away at each other for the entirety of the war. The action centered on Ypres (above and to the left of the center of the box), which is where the Allies dug in to hold off the German advance.

Ypres stood in the way of the German plan to sweep through Belgium into France. The contest for the city is one of history's greatest stalemates. Over the course of the war, millions of men died in miles of trenches dug here.

Aerial oblique photos like this weren't as tactically useful as those that looked straight down, but they were easier for laymen to read, and were used for infantry to visualize their assault. This photo is of Ypres in 1915.

Two soldiers from the US Army's Signal Corps attach camera to their plane's fuselage. This camera probably had a focal length of around 50 centimeters, which gave a good balance of detail and area.

Passchendaele was a small town near Ypres on the German side of the front lines. In the summer of 1917, the Allies launched an offensive on the town because they saw it as vital way to disrupt the German supply chain.

Rain and mud caused the battle to drag on for months. By the end, the small town had been bombed to oblivion. The total fatalities are disputed, but with anywhere from 500,000 to 1 million dead, the Battle of Passchendaele became one of the war's most infamous battles. A few walls of the church were all that remained.

The Great War Seen From The Air includes colorful icons that highlight supply routes, trench lines, and other things of strategic importance.

This smoke isn't from bombs, it's from generators. The Germans used smoke to conceal their artillery from aerial reconnaissance. This photo was taken over Ostend, a coastal Flemish city.

Trenches have a distinctive zig zag pattern to minimize the damage a single bullet or explosion could cause. This photo also shows the scale of devastation that bombing left on the landscape.

Both sides used aerial photography to keep their tactical maps up to date. This is a British trench map of an area south of Ypres from September 1915. The blue markings are Allied resources, and the red lines are German trenches.

This photo of a German Albatros C.III reconnaissance plane was taken in 1916 from a Belgian plane. This was both strategically and technically difficult. In addition to the inherent danger of photographing an enemy (who was equipped with a machine gun), the photographer had to manually focus using a tiny aperture all from a jumpy little plane several thousand feet above ground.