Italy declares neutrality. World War II officially begins. September 3 - England, France, Australia and New Zealand declare war on Germany. September 5 - Two Neutrality Proclamations issued by President Roosevelt. Roosevelt also orders Navy to perform a neutrality patrol to track any belligerent naval force approaching the U.S.'s Atlantic coast or the West Indies.
German troops cross the Vistula River in Poland.
September 6 - Neutrality patrol for Atlantic ocean formed under Rear Admiral A.W. Johnson, Commander Atlantic Squadron. September 8 - Roosevelt declares a limited national emergency and increases the enlisted strengths of all armed forces and authorizes retired officers, men and nurses to be recalled to active duty to the Navy and Marines. Long range blockade of Germany announced by Allies. September 10 - Canada declares war on Germany. September 16 - British establish convoy system for merchant shipping.