I found a mistake in The Great Impersonation

Germany has just invaded Poland, and Great Britain has declared war on Germany. A German nobleman in the diplomatic corps in Africa happens upon an old school friend whom he favors enough to be his twin, so he goes to Berlin get approval to impersonate the Englishman. He starts to play the part, and gives his superior details of the life of the Englishman and says, "My father was killed during the last few days of the First World War." But, of course, at that point it was not the "first" world war, but the only world war, because the second had just begun, but at that point no one could know for sure that it would turn into a conflict of that magnitude.