<P> As with most technologies, aircraft and their use underwent many improvements during World War I. As the initial war of movement on the Western Front settled into trench warfare, aerial reconnaissance over the front added to the difficulty of mounting surprise attacks against entrenched and concealed defenders . </P> <P> Manned observation balloons floating high above the trenches were used as stationary observation posts, reporting enemy troop positions and directing artillery fire . Balloons commonly had a crew of two, each equipped with parachutes: upon an enemy air attack on the flammable balloon, the crew would jump to safety . At the time, parachutes were too heavy to be used by pilots in aircraft, and smaller versions would not be developed until the end of the war . (In the British case, there arose concerns that they might undermine morale, effectively encouraging cowardice .) Recognized for their value as observer platforms, observation balloons were important targets of enemy aircraft . To defend against air attack, they were heavily protected by large concentrations of antiaircraft guns and patrolled by friendly aircraft . </P> <P> While early air spotters were unarmed, they soon began firing at each other with handheld weapons . An arms race commenced, quickly leading to increasingly agile planes equipped with machine guns . A key innovation was the interrupter gear, a Dutch invention that allowed a machine gun to be mounted behind the propeller so the pilot could fire directly ahead, along the plane's flight path . </P> <P> As the stalemate developed on the ground, with both sides unable to advance even a few miles without a major battle and thousands of casualties, planes became greatly valued for their role gathering intelligence on enemy positions . They also bombed enemy supplies behind the trench lines, in the manner of later attack aircraft . Large planes with a pilot and an observer were used to reconnoiter enemy positions and bomb their supply bases . These large and slow planes made easy targets for enemy fighter planes, who in turn were met by fighter escorts and spectacular aerial dogfights . </P>

How did trench warfare affect the movement and pace of world war 1