On 1 September 1939 Hitler ordered the invasion of Poland. An ultimatum was issued by Britain that German troops should retreat, but when it went unanswered the Second World War erupted on 3 September. From the perspective of the Soviet Union, the Second World War breaks down into two distinct phases: the period between the conclusion of the Nazi-Soviet Pact and the German invasion of the Soviet Union in the summer of 1941; and the Great Patriotic War fought against Germany from June 1941 to May 1945. During the ﬁrst phase, the Soviets annexed territory under the conditions of the Secret Protocol to the Nazi-Soviet Pact, which included waging war against Finland, and engaged in trade and further negotiations with Nazi Germany. After June 1941 they were locked in a ﬁerce conﬂict on the Eastern Front as well as being members of the Grand Alliance with Britain, France and the United States.