美国参加“二战”后，丘吉尔说了一段意味深长的话：“假使当初我们采取共同的措施来保障我们的安全的话，这个灾难，本来永远不会降落到我们的头上。”结合你对“二战”的起因和英美苏参战的思考，谈谈你对这段话的理解。
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