Other mistake: Dante and The Agency both have extensive photos and videos of the team's previous exploits. Trouble is that most of it is simply stills and footage from the previous movies in the franchise, often occasions when nobody was around with a camera to witness and document what was going on, certainly not at the angles we see.
Answer: These were barrage balloons, commonly used during the war. They are used to stop low level bombing and low level fly bys by enemy fighter planes. The cables attached to the balloons are designed to cut through the wings of the aircraft and to bring them down, so any pilot would have to fly above them, and the balloons would also restrict the view from above.