I believe its RADAR.
They had airplanes in world war 1.
Same with machine guns.
And the first repeating rifle was made in 1860.
Which leaves the Radar.
-Steel jelly
(The height of the Japanese atrocity in China was reached at the Rape of) D: Nanjing
People began to be More Settled and more like Modern day people they began to make laws and other ways of communication because a civilization is nothing with out it .