关于世界反法西斯同盟的建立，请回答以下问题：
（1）1942年，反法西斯同盟得以建立的关键因素有哪些？
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（2）反法西斯同盟成立之前和之后，对战争产生了怎样的影响？
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（3）从上述历史过程中，我们得到的历史启示是什么？
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