<P> In the end, the war ended through a combination of attrition (of men and material), advances on the battlefield, arrival of American troops in large numbers, and a breakdown of morale and productivity on the German home - front due to an effective naval blockade of her seaports . </P> <P> As with most other technologies, the aircraft underwent many improvements during World War I . </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 and bombing the enemy's supplies behind the trench lines . Large planes with a pilot and an observer were used to reconnoiter enemy positions and bomb their supply bases . Because they were large and slow, these planes made easy targets for enemy fighter planes . As a result, both sides used fighter aircraft to both attack the enemy's observer planes and protect their own . </P>

What specific invention made trench warfare a necessity in ww1