The answer should be 12 million.
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It would prove to be extremely ineffective. When the United States decided not to join the league, it was primarily made up of weak countries so it didn't have any muscle to enforce anything. This was proven when Japan inbaded Manchuria despite the league trekking then not to. This showed Hitler that without the united States in the league of Nations, there would be nobody to step in and stop Germany from invading Poland in 1939
One problem was that some states didn't fully understand the meaning for it