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Nazi Germany, the Slovak Republic, and the Soviet Union -
marking the beginning of World War II -
Germany invaded the Netherlands, Belgium, Luxembourg, and France. Luxembourg was occupied that same day. The Netherlands surrendered on 15 May, Belgium on the 28th. At first, Great Britain supported the Netherlands, Belgium, and France, but it withdrew later -
If the BEF had been captured, it would have meant the loss of Britain's only trained troops and the collapse of the Allied cause