Why did the invasion of Poland become a decisive event in World War II?

Correct answer: After it Britain and France declared war against Germany

The invasion of Poland was decisive because it turned the European crisis into an open international war. Germany attacked Poland on 1 September 1939, using force to expand its territory. Britain and France had promised support to Poland and, after the attack, declared war on Germany. This made the conflict much wider than a dispute between Germany and Poland. Therefore, option A is correct. The other choices describe unrelated events: the United Nations did not end, the French Revolution had occurred much earlier, and America had become independent in the eighteenth century.

Remember that the invasion itself did not mean that every country immediately joined the war, but it was the direct trigger for the declarations by Britain and France. It is commonly treated as the beginning of the Second World War in Europe. The date 1939 is useful for examination revision. The key chain is: German invasion of Poland, British and French declarations of war, and the expansion of the conflict into World War II. Thus the supplied answer is accurate.