There is the landmark.

Artillery observation was used for the first time at the Battle of the Aisne, in the German retreat from the Marne.

Thenceforward, month by month, the men in the clouds became increasingly the indispensable guides and allies of the men on the ground, searching out and signalling the guns of the enemy, while preventing his fliers from searching out and signalling our own.

Next came the marvellous development of aerial photography, by which the whole trench world, the artillery positions and _hinterland_ of the hostile army could be mapped day by day for the information of those attacking it; the development of the bombing squadrons, which began by harassing the enemy's communications immediately behind the fighting line, and developed into those formidable expeditions of the Independent Force into Germany itself, which so largely influenced the later months of the war.

Finally, the airman, not content with his own perpetual and deadly fighting in the air, fighting in which the combatants of all nations developed a daring beyond the dreams of any earlier world, began to take part in the actual land-battle itself, swooping on reserves, firing into troops on the march, or bringing up ammunition.
[12] From the recent Official Report issued by the Air Board.
And while the flying Army of the Winds was there developing, the flying Army of the Seas, its twin brother, was not a whit behind.