Why did great britain and france finally declare war on germany in September 1939

The first casualty of that declaration was not German-but the British ocean liner Athenia, which was sunk by a German U-30 submarine that had assumed the liner was armed and belligerent. There were more than 1,100 passengers on board, 112 of whom lost their lives. Of those, 28 were Americans, but President Roosevelt was unfazed by the tragedy, declaring that no one was to "thoughtlessly or falsely talk of America sending its armies to European fields." The United States would remain neutral.