For World War I pilots, the most terrifying song that relates to their experience may not be Seven Nation Army but 99 Luftballoons, because going against barrage and observation balloons in the Great War was terrifying.

Pilots with the balls and skill to attack these balloons were known as balloon busters, and ones that had shot down more than five of the balloons were known as balloon aces. And yes, shooting down a balloon counted as a "kill," same as shooting down a piloted enemy plane. But what made them so hard to shoot down? After all, they were just a bunch of floating bags of air. Pop 'em with a needle and get on with your day, right? Well, no. First, military balloons weren't made of cheap Mylar or latex. Many in World War I were made of tightly woven fabric, though vulcanized rubber and Thiokol rubber were prominent in World War II. All of these materials could take plenty of hits without splitting, meaning bullets that passed through them caused them to leak instead of to pop.

A row of spherical barrage balloons used for suspending aerial nets (Australian War Memorial) So they couldn't simply be popped, and it often took a lot of rounds to bring one down. But if a fighter did manage to slay the beast, he wasn't out of danger yet. While American balloons in World War II were sometimes filled with helium, none of the early Great War combatants had access to that gas, and hydrogen was the preferred gas for barrage balloons anyway. Why? Well, for the same reason it was bad for the Hindenburg. Observation balloons had people in them, people who would've loved helium instead of hydrogen over their heads. But barrage balloons were empty, and filling them with hydrogen meant that, when destroyed, the balloons had a tendency to go out in massive fireballs. This was a huge threat to the fighters attacking it. It also meant that fighters had one advantage though: Incendiary rounds were very effective against the balloons. But in World War I, pretty much only the British had incendiary rounds in planes. Everyone else was slinging cold metal. And incendiary rounds didn't stay hot forever, generally traveling only 300 to 400 yards while still burning. You did not want to be 300 yards from an exploding balloon and still flying towards it as you would have to be to effectively shoot at it.

Barrage balloons and their crews in World War II. (Royal Air Force) Fine, fine, fine. The balloons were hard to shoot down and, when shot down, might explode in a big fireball and kill the attacking fighter. Fine. Just fly around them, right? Let the Germans have their balloons over their lines, maybe bring in some air defense artillery to shoot at it. But let the fighters avoid them. Nope. For two reasons. First, those observation balloons were an enduring threat from the moment they went up until the moment they went down. Artillery observers sat in them and reported troops positions and movements to their friendly artillery for hours, allowing German crews to hit English, French, and U.S. positions all day. They had to be killed. But the barrage balloons couldn't be ignored either, because they had thick steel cables or else entire nets hanging from them in order to catch enemy fighters attempting to fly under them. And they flew high enough that few World War I fighters or bombers could come over the top and still be effective. By World War II, the balloons were set lower, but only to steer the enemy aircraft up to over 5,000 feet where anti-aircraft artillery was most effective.

American pilot Frank Luke poses with his 13th confirmed kill. (Public Domain) So observation balloons and barrage balloons were lethal, terrifying, and absolutely had to be destroyed, and some of America, England, and France's top aces proved their mettle by flying at the things, especially in World War I. In fact, some of the top decorated fighter pilots of World War I had few wins against human-piloted planes, but a dozen or more against balloons. Will Coppens, a Belgian pilot, personally awarded a medal by King Albert I had only shot down two enemy planes in his career, but he had taken down an astounding 35 enemy balloons. The next highest scoring pilot after him was Frenchman Leon Bourjade with 27. So, yeah, Coppens earned that medal from his king. America's top balloon buster was Frank Luke, a mouthy pilot who was looked down upon by his peers when he arrived in France. He claimed his first fighter kill in August 1918, but no one else had witnessed the feat, and he was written off as a blowhard. So, after hearing how hard balloons were to take down, he attacked one on September 12 and, after three passes, destroyed it right before it reached the safety of the ground where the observers could clamber out.