A large-scale war that later became known as the Second World War started in Europe on 1 September 1939 with the German invasion of Poland. The United Kingdom and France declared war on Germany on 3 September. The Allies assessed that they needed time to equip their armies. Until May 1940, the war was mainly fought over maritime routes. The Western powers sought to prevent maritime transport to Germany with their surface fleets. In turn, Germany sought to isolate the United Kingdom with submarines, as it had done during the First World War. Aerial warfare had yet to find its form.
Germany invaded Poland on 1 September 1939 without having declared war
Germany launched its attack on Poland in the early morning of 1 September 1939. The attack started without a declaration of war. Chancellor of Germany Adolf Hitler made a speech at the German Parliament (Reichstag) on the morning of 1 September. It was broadcast on the radio. In his speech, Hitler announced that Germany had responded to Polish fire.
Germany’s invasion led to the Western powers declaring war on 3 September
Italy’s Duce Benito Mussolini declared on 2 September that Italy was a non-belligerent. Over the course of the following days, Mussolini sought to initiate negotiations regarding the situation of Poland. The absolute requirement set by the United Kingdom and France for negotiations was that Germany quickly pull its forces out of Poland. The United Kingdom and France declared war on Germany on 3 September. Over the course of September, the Commonwealth of Nations also declared war on Germany. Many countries, including Belgium, the Netherlands, the Soviet Union, the United States of America, Denmark, Norway, Sweden and Finland, declared themselves to be neutral.
The United Kingdom and France issued their ultimatums before declaring war
The declarations of war by the United Kingdom and France were preceded by ultimatums issued by both countries to Germany on 3 September. These two Western powers had guaranteed the inviolability of the Polish borders in March 1939 and agreed to come to its aid in the event of an attack against Poland by any European country. Similar guarantees had also been made to Romania.
A note submitted by the United Kingdom in Berlin on the morning of 3 September stated that the countries would be at war with each other unless Germany aborted its military operations and pulled back from the areas of Poland it had conquered by 11am the same day. A similar ultimatum issued by France set the deadline for the withdrawal to 5pm on 3 September.
The United Kingdom passed a bill on conscription on 3 September
The United Kingdom had a recruited army. In spring, the country’s government had brought before Parliament a partial conscription bill. According to the bill, those between the ages of 20 and 22 could be called to service for a period of six months. Parliament passed an amended version of the bill on the day that war was declared. According to the new law, all men between the ages of 18 and 41 could be called to service.
A war without ground and air operations
The United Kingdom and France judged that they needed time to arm their armies and create the war industry production that this armament required. The Western powers decided to concentrate on defence for the time being. They estimated that it might take a long time to make their military power stronger than that of the enemy. At the same time, the German war industry was increasing Germany’s own military power. The Western powers had to hinder this by blocking Germany’s access to raw materials.
After declaring war, the United Kingdom and France did not launch significant ground or air operations against Germany. During the invasion of Poland, Hitler forbade the forces deployed near Germany’s western border from crossing the border, and he also limited the operations of the German Air Force (Luftwaffe).
The French ground forces remained on the defensive
Rather than take action, the Allies waited for Germany to attack. The French Army was concentrated in a defensive formation. Over the course of the autumn, both British and Canadian troops were deployed to France.
The French Army advanced to the front of Germany’s western front in Saarland. The German covering forces retreated to a fortified area. The French forces returned to their starting positions on 17 October. The situation at Germany’s western front remained stable with regard to the forces’ operations until May 1940.
France decided to start the war with a naval blockade
The main means used by the Western powers to weaken Germany was blocking its maritime transport operations with a naval blockade. Its effect was weakened by the neutrality of the Soviet Union, which was accommodating towards Germany. Germany was able to purchase many of the materials it required for its war industry from the Soviet Union. This improved the ability of Germany and its war industry to endure even a prolonged naval blockade.
The second key hinderer of the naval blockade was Italy. Following the German invasion of Poland, Italy had declared itself to be a non-belligerent. The Allies wanted it to maintain this status and wanted to refrain from provoking it. However, Italy was contractually tied to Germany. Italy’s trade with Germany supported the latter’s war efforts.
The strategy chosen for the ground and air forces meant strengthening France’s defences with British and Canadian forces. The defensive preparations in France were weakened by Belgium’s neutrality. The French–Belgian border was unfortified. The Allies would have liked to concentrate their troops in Belgian territory.
The Battle of the Atlantic started on 3 September 1939
The declarations of war by the United Kingdom and France were followed by mutual naval blockades. The Battle of the Atlantic started on 3 September and lasted until Germany’s collapse in May 1945. Both parties tried to block each other’s maritime transport, with the United Kingdom using surface vessels and Germany using submarines.
The Allies lacked a centralised military command
The United Kingdom and France established the Anglo-French Supreme War Council to coordinate the political decisions related to warfare. The strategic decisions pertaining to the war were first prepared in both countries and later decided upon at meetings of the Supreme War Council.
Military plans were prepared by each country’s military headquarters. The coordination was handled at joint meetings of both countries’ chiefs of staff.
The Soviet Union invaded Poland on 17 September
The German forces quickly defeated the Polish forces and advanced to the border of the Soviet and German spheres of influence in Poland, as specified in the secret supplementary protocol of the Molotov–Ribbentrop Pact, signed on 23 August 1939. The Red Army launched its own invasion on 17 September and conquered the eastern parts of Poland. Germany and the Soviet Union amended their agreed spheres of influence with an agreement that they signed on 28 September.
Germany made a peace offer to the Allies at the beginning of October
After Germany conquered Poland, Hitler made a speech in the German Parliament (Reichstag) on 6 October, announcing that Germany was ready to make peace with the Allies. British Prime Minister Neville Chamberlain rejected the possibility of peace in a speech he made on 12 October.
The start date of Germany’s attack to the west was changed 29 times
Hitler had already signed an order on preparations for an attack to the west on 9 October 1939 in case his offer to end the war was rejected. Colonel General Walther von Brauchitsch, the commander-in-chief of the German Army, supervised the attack preparations. He assessed that the German Army was not yet ready to launch the attack. Under pressure from Hitler, von Brauchitsch yielded during negotiations held with him on 5 November that the attack to the west would start on 12 November. The date of the attack was moved for the first time on 7 November and then afterwards another 28 times. The attack started on 20 May 1940.