On April 22, 1915, German forces shock Allied soldiers along the western front by firing more than 150 tons of lethal chlorine gas against two French colonial divisions at Ypres, Belgium. This was the first major gas attack by the Germans, and it devastated the Allied line.
https://www.history.com/this-day-in-history/germans-introduce-poison-gas
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Because of the lack of US involvement in the world matters, the rising tensions in the world and because of the inaction and lack of willingness to deal with the problems
The League of Nations were formed shortly after the end of WWI and the United States went back to the policy of isolationism and the allies adopted a policy of appeasement and neutrality which further gave the impression that it was allowed to do whatever they wanted.
I would have made a treaty with them stating that the reservations werent need they could have moved to a different part of their land and they could have avoided the violence.
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George Washington appointed Nathanael Greene and commander in the South
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