Please clarify your question about "so many nations" joining the allies after Pearl Harbor.

Most of the allies had been in the war well before Pearl Harbor, most since September 1939 and more European countries followed as they were attacked by Germany.

The United States of America declared war and joined the allies in December 1941 after Pearl Harbor because they were attacked by Japan, and Germany and Italy declared war on the USA. Cuba declared war in December 1941 joining in support of the USA, Mexico in 1942 after having her shipping (to the USA) attacked by German submarines and Brazil joined in 1942 after allowing American bases and subsequently having her shipping attacked by German and Italian vessels.