Airplanes in WWI. I know they were pretty new then,

[eppur_se_muova]

6. Don't forget that German Zeppelins bombed several cities, including London ....

and the Brits eventually learned how to shoot them down with incendiary bullets fired from airplanes.

As several others have noted, their first real use was reconnaisance. Then one of the pilots fired a gun at an enemy pilot, and things took off after that.

Supply convoys, including trains, were probably more vulnerable to strafing than were trenches. Hitting artillery emplacements before an attempted advance would have been an obvious move as well.