Diversification is important in investing

It is important because it aims to maximize return by  investing  in different areas that would each react differently to the same event most  investment  professionals agree that although it does not guarantee against loss diversification  is the most  important  component of reaching long-range financial goals while minimizing risk

I think the Berliners knew about the air strike because of the agreement that Poland and Great Britain had signed. In that agreement, it states that if Poland was to be invaded by any nation Britain will provide military support for them.