This is a timeline of events of World War II in 1939 from the start of the war on 1 September. For prior events, see the timeline of events preceding World War II.

Germany's invasion of Poland on 1 September 1939 brought many countries into the war. This event, and the declaration of war by France and Britain two days later, mark the beginning of World War II. After the declaration of war, Western Europe saw minimal land and air warfare, leading to this time period being termed the "Phoney War". At sea, this time period saw the opening stages of the Battle of the Atlantic.

In eastern Europe, however, the agreement between the Soviet Union and Nazi Germany signed on 23 August opened the way in September for the Soviet Union's invasion of eastern Poland, which was divided between the two countries before the end of the month. The Soviet Union began a new military offensive by invading Finland at the end of November. France and Britain provided minimal physical support to Poland during the invasion.

The war in East Asia among the Republic of China and the Empire of Japan reached a stalemate, while increasing clashes between Japan and the Soviet Union ended when the two parties agreed in September on a ceasefire.

September

1 September The Republic of China and the Empire of Japan are involved in the early stages of the third year of armed conflict between them during the Second Sino-Japanese War. The war is in what will be known as the "Second Period", which begins after the fall of Wuhan in October 1938 and ends in December 1941 with Pearl Harbor. This conflict will eventually be swept up into World War II when Japan joins the Axis and China joins the Allies. The invasion of Poland by Germany starts at 4:45 am when the Kriegsmarine'''s battleship Schleswig-Holstein opens fire on the Polish military transit depot at Westerplatte in the Free City of Danzig on the Baltic Sea, but the attack is repulsed. At the same time the Luftwaffe attacks several targets in Poland, among them Wieluń, the first town in the war to be carpet bombed by the Germans. Shortly before 6:00 a.m., the German Army passes the Polish border in great numbers from north and south, together with Slovak units. On the same day, the Free City of Danzig is annexed by Germany. Resisters entrenched in Danzig's Polish Post Office are overwhelmed. Adolf Hitler cites alleged Polish border attacks that happened during the false flag Operation Himmler as a reason for war during his 1 September 1939 Reichstag speech. The Italian government announces that it will maintain a condition of "non-belligerence" in the conflict. Denmark, Estonia, Finland, Latvia, Norway and Sweden immediately declare their neutrality. Portugal proclaims that it will remain neutral in the war. The House of Commons of the United Kingdom passes an emergency military budget. The British War Secretary Leslie Hore-Belisha orders the War Office to begin the general mobilization of the British Armed Forces. In a mass evacuation effort (code named "operation Pied Piper") the British authorities relocate 1,473,000 children and adults from the cities to the countryside. The adults involved were teachers, people with disabilities and their helpers, mothers with preschool children. Acting on account of their governments, the ambassadors of France and Britain demand the German government to cease all hostile activities and to withdraw its troops from Poland. The President of the United States Franklin Delano Roosevelt sends an appeal to all European powers involved in the crisis asking them to abstain from bombing civilian and unfortified cities. Germany's Führer, Adolf Hitler, answers immediately assuring the American chargé d'affaires Alexander C. Kirk that the Luftwaffe will only attack military targets. The British Prime Minister Neville Chamberlain also promises to abide to the request, as does Poland's ambassador to the US Jerzy Antoni Potocki. The Einsatzgruppen initiate the Operation Tannenberg in Poland, which would kill around 20,000 selected Poles in two months. Wartime blackouts are put in place throughout Britain, Germany and France. The Slovak State suspends the country's civil liberties.

2 September Right after Britain, the French Parliament also approves an emergency war budget. The British and French governments agree on issuing an ultimatum to Germany the following day. The Swiss government proceeds to a general mobilization of its forces. The Irish State's Dáil Éireann approves a state of emergency, paving the way to legislation that vastly enhances the government's powers. The French Army begins its general mobilization.

3 September At 9:00 a.m. the British ambassador to Berlin Nevile Henderson is instructed by the Cabinet to deliver an ultimatum to Germany which expired without answer at 11:00 a.m. At 11:15 a.m. British Standard Time (BST) the Prime Minister Neville Chamberlain announces to the public that Britain is at war with Germany. The National Service (Armed Forces) Act 1939 is approved and enforces full conscription in the British Armed Forces on all able-bodied males between 18 and 41 resident in the UK. In Britain, Chamberlain forms a new war ministry with a smaller and more powerful war cabinet within composed of nine ministers (Chamberlain, Sir Samuel Hoare, Sir John Simon, Lord Halifax, Leslie Hore-Belisha, Sir Kingsley Wood, Lord Chatfield, Lord Hankey and Winston Churchill). During its first meeting, Britain's war cabinet appoints general Sir Edmund Ironside as head of the Chief of the Imperial General Staff and general Viscount Gort head of the British Expeditionary Force. The British Viceroy of India Lord Linlithgow also declares war on Germany without consulting Indian nationalists. The Australian Prime Minister Robert Menzies declares that the country is at war with Germany due to Britain's choice, and a similar war declaration against Germany is made by New Zealand's government. Newfoundland declares war on Germany. At 12:00 p.m. the French Government delivers a similar final ultimatum to Germany which at 5:00 p.m. also expires unanswered, thus bringing France in the war. Within hours of the British declaration of War, , a British cruise ship en route from Glasgow, UK, to Montreal, Canada, is torpedoed by the German submarine Northwest of Ireland. 112 passengers and crew members are killed. The "Battle of the Atlantic" starts. "Bloody Sunday": accused of having shot at Polish troops, about 1,000 ethnic German civilians are killed in the Polish city of Bydgoszcz. Ireland's Taoiseach Éamon de Valera declares the nation's neutrality. Netherlands and Belgium declare their neutrality. German authorities order U-boats to immediately take action against all British ships, but sparing French ships and in strict observance of prize rules. The Polish destroyer ORP Wicher and the minelayer ORP Gryf are sunk in the Polish port of Hel by the Luftwaffe, making them the first warships to be sunk in the war. In Britain's first military action, the Royal Air Force's Bomber Command sends out 27 planes to bomb the Kriegsmarine, but they turn back before having been able to find any targets. Overnight ten Whitleys made the first of many 'nickel raids' in Bremen, Hamburg and the Ruhr in which the planes dropped propaganda leaflets. Further answering to Roosevelt's plea the British and French present a joint formal declaration stating that the Allied bombers would attack only military targets unless Germany begins indiscriminate civilian bombings. Poland permits Czech and Slovak refugees to create a Czechoslovak Legion to fight Germany.

4 September In Poland the Third German Army from East Prussia links with units from German Western Pomerania, thus covering the Danzig Corridor. In the first British raid of the war, the Royal Air Force's send 15 Blenheim bombers to launch a bombing raid on the German fleet in the Heligoland Bight. They target the German pocket-battleship Admiral Scheer and the light cruiser Emden anchored off Wilhelmshaven. Seven aircraft are lost in the attack and, although the Admiral Scheer is hit three times, all of the bombs fail to explode. Japan's Prime Minister Nobuyuki Abe sends a formal note to all belligerents and neutrals announcing it would remain neutral and "avoid becoming involved" in the European conflict; instead it will concentrate on "settling the China incident". Spain's caudillo Francisco Franco states that he will observe "strict neutrality" in the conflict. Lithuania proclaims its neutrality in the conflict. The Imperial State of Iran officially announces its intention to remain neutral in the war. The South African Prime Minister Barry Hertzog motion to remain neutral in the war is defeated in the Assembly 80 votes against 67. At this point, Hertzog goes to the Governor-General Patrick Duncan and asks him to call a new election, which Duncan refuses. The first advance parties of the British Expeditionary Force arrive in France. After the sinking of the Athenia Hitler forbids any attack on passenger ships. Argentina, Brazil and Mexico proclaim their intention to remain neutral in the European war.

5 September The National Registration Act 1939 is passed in Britain permitting to establish a register on the whole population. Duncan calls on the politician Jan Smuts to attempt to form a Cabinet and replace Hertzog as Prime Minister of South Africa, which he successfully does. The Kingdom of Yugoslavia states its neutrality. The British freighter SS Bosnia becomes the first merchant ship sunk in the battle of the Atlantic when it gets targeted off the coast of Portugal by the U-boat U-47. The United States publicly declares neutrality. Following the administration's declaration of neutrality, American President Franklin D. Roosevelt orders to put together a Neutrality Patrol which must observe and report any belligerent forces by patrolling the United States Atlantic coast and the Caribbean. Following a mutual request by Argentina, Brazil, Chile, Colombia, Cuba, Mexico, Peru, Venezuela and the United States, Panama sends invitations for a conference of American foreign ministers to be held at Panama City on the current war. The Kingdom of Iraq cuts its diplomatic relations with Germany. Britain terminates its diplomatic recognition of Slovakia. Germany's Foreign Minister Joachim von Ribbentrop makes clear that no hostile actions against Romania will be tolerated by the Reich.

6 September South Africa, now under Prime Minister Jan Smuts, declares war on Germany. The Kingdom of Egypt cuts its diplomatic ties with Germany. In the so-called battle of Barking Creek, a friendly fire incident, due to the misidentification as hostile of an incoming team of eleven Hurricanes, two aircraft are shot down and the first British fighter pilot killed. In southern Poland The 14th German army takes Kraków virtually unopposed. As a protection against U-boats, the Admiralty orders the adoption of the convoy system. The British fleet starts the naval blockade on shipping directed to Germany by the implementation of the Northern Patrol. The Crown Council of Romania votes in favour of the country's neutrality in the war.

7 September: France's commander in chief general Maurice Gamelin begins a limited offensive into the German Saarland territory involving ten divisions.

8 September Britain establishes the Ministry of Food to monitor the supply and distribution of food. Roosevelt proclaims "a limited national emergency", increasing military spending and expanding the size of the United States Armed Forces. The Germans begin what will be the systematic mining of the British waters by the mining of Portland Harbour. Britain formally announces that Slovakia is to be considered a territory under German occupation.

10 September