From Square Enix:
It is 11941 A.D. Years before the events of Nier: Automata, the last remnants of humanity have fled Earth to seek refuge on the Moon. At the same time, the Android fighters known as the YoRHa continue to wage their millennia-long war against the alien Machines in humanity's place. When sixteen YorHA soldiers are sent to the ruins of Pearl Harbor to destroy a looming Machine threat, disaster strikes, and only four survivors remain to finish the job.
The units designated No. 2, No. 4, No. 16, and No. 21. When the squad encounters another, older band of Androids, the lost deserters of another long-ago failed mission, the YoRHa soldiers' battle for survival becomes even more complicated, forcing No. 2 and her sisters-in-arms to reckon with their war's most dangerous and damning secrets.

Based on the stage play “YoRHa“, first performed in 2014, that forms the backstory of the 2017 game Nier: Automata.