It's is not often you stump Ernie, but JonnyS wins the prize! Parachute is the answer...
Parachutes had been used by balloon observers for at least two years before the were used with aircrew.
The Central Powers began using the parachute for pilots in 1918. While the Allies had the technology the powers that be decided that having an ÔÇśeasyÔÇÖ way out would reduce the ÔÇśfighting spiritÔÇÖ of the pilots. The same leadership thought that 60,000 casualties on the first day of the Battle of the Somme was a reasonable result. Compared to that, what are a few hundred? pilots/observers in the great scheme of things.
The parachutes used had fixed lines and as a result the pilot sometimes got stuck in the rigging and plummeted to his death but compared with jumping sans chute or blowing you brains out if the plane was on fire it represented a major leap forwards and could be argued to give the pilot a more aggressive spirit than without it.
Hermann G├Âring, would have been a footnote in history (and possible a question on this forum) if had not survived ÔÇśballing outÔÇÖ in 1918. Without his support in the 1920ÔÇÖs a certain corporal might never have been seen in public, without his intervention, the Battle of Britain may have had a very different outcome.
Irvine developed the deployable parachute which saves thousands of aircrew over the the course of WWII and since. Not to mention recent world records.
Paratroop assault troops were first introduce by the Soviet Union, though most general staff that advocated such mobile tactics were 'purged' in the 1930s. The Axis and Allies had major successes and failures with airborne assaults.