What caused industrial psychologists to begin working with the military?

What caused industrial psychologists to begin working with the military was U.S. entry into WWI.
After years of upholding its isolationist policy of not getting involved with any wars it has nothing to do with, the US actually decided to go back on that decision and join the WWI. This is why many psychologists started working with the army in order to help the soldiers.

To claim new land in the new world

In 911, a group of vikings lead by rollo besieged paris and chartres. after a victory near chartres on 26 august, charles decided to negotiate with rollo. the talks were led by hervé, the archbishop of reims. this resulted in the treaty of saint-clair-sur-epte, which granted rollo and his soldiers all the land between the river epte and the sea. it also granted him brittany, "for his livelihood". at the time, brittany was an independent land which france had not been able to conquer. in exchange, rollo promised the king his loyalty and military assistance when needed. also rollo also agreed to be baptised and to marry gisela, the king's daughter. the territory given rollo is about the same as upper normandy.