Title: Military history of the United Kingdom during World War II
Content:  The military history of the United Kingdom in World War II covers the Second World War against the Axis powers, starting on 3 September 1939 with the declaration of war by the United Kingdom and France, followed by the UK's Dominions and Crown colonies, on Nazi Germany in response to the invasion of Poland by Germany. There was little, however, the Anglo-French alliance could do or did do to help Poland. The Phoney War culminated in April 1940 with the German invasion of Denmark and Norway. Winston Churchill became prime minister and head of a coalition government in May 1940. The 
Title: World War II by country
Content:   The United Kingdom, along with most of its Dominions and Crown colonies, declared war on Nazi Germany in September 1939 to honour its commitments to Poland after the German invasion. After the fall of France, Britain was the only major Allied nation left in Europe. In the early war, the Royal Navy engaged Axis ships in the Battles of the Atlantic and the Mediterranean, while the army fought in the Balkans, East African, and Western Desert Campaigns. Britain entered the Pacific Ocean theater of World War II when Japan seized many British colonies in Asia in 1941–1942. After the Soviet Union and United States joined the Allies in 1941, the U.K. launched major offensives in North Africa, Italy, and Normandy. As a member of the Big Three, Prime Minister Winston Churchill participated in conferences 
Title: British Armed Forces
Content:  War, by 1933 fascism had given rise to Nazi Germany, which under the leadership of Adolf Hitler re-militarised in defiance of the Treaty of Versailles. Once again tensions accumulated in European relations, and following Germany's invasion of Poland in September 1939, the Second World War began (1939–1945). The conflict was the most widespread in British history, with British Empire and Commonwealth troops fighting in campaigns from Europe and North Africa, to the Middle East and the Far East. Approximately 390,000 British Empire and Commonwealth troops lost their lives. Allied victory resulted in the defeat of the Axis powers and the establishment of the United Nations (replacing the League of nations).
Title: Kenya in World War II
Content:   The involvement of the British Colony of Kenya in World War II (Vita Kuu ya Pili ya Dunia) began with the declaration of war on Nazi Germany by the British Empire in September 1939. Though some fighting with Italian troops occurred in Kenya itself from June 1940 to February 1941, it remained an important economic asset for the Allies and also contributed a significant number of soldiers to fight in the British Army.
Title: United Kingdom
Content:  culminated in the General Strike of 1926. Britain had still not recovered from the effects of the war when the Great Depression (1929–1932) occurred. This led to considerable unemployment and hardship in the old industrial areas, as well as political and social unrest in the 1930s, with rising membership in communist and socialist parties. A coalition government was formed in 1931. Nonetheless, "Britain was a very wealthy country, formidable in arms, ruthless in pursuit of its interests and sitting at the heart of a global production system." After Nazi Germany invaded Poland, Britain entered the Second World War by 
Title: Military history of the United Kingdom during World War II
Content:  The attack on Pearl Harbor in December 1941 and the subsequent German declaration of war on the United States made it a global war. German U-boats conducted a highly successful campaign against traffic along the American east coast. A proportion of the ships sunk were en route to assembly points for convoys to the UK. German sailors called this the "second happy time". It came to an end when a convoy system operated along the coast and adequate anti-submarine measures were employed.
Title: British entry into World War I
Content:  ''' Great Britain entered World War I on 4 August 1914 when the King declared war after the expiration of an ultimatum to Germany. The official explanation focused on protecting Belgium as a neutral country; the main reason, however, was to prevent a French defeat that would have left Germany in control of Western Europe. The Liberal Party was in power with prime minister H. H. Asquith and foreign minister Edward Grey leading the way. The Liberal cabinet made the decision, although the party had been strongly anti-war until the last minute. The Conservative Party was pro-war. The Liberals knew that if they split on the war issue, they would lose control of the government to the Conservatives.
Title: Timeline of the British Army
Content: 1939 ; 3 September 1939 – Britain enters the Second World War when it declares war, along with its Allies, on Nazi Germany. ; September 1939 – British Expeditionary Force begins to land in France. ; 17 May 1940 – The Local Defence Volunteers (later the Home Guard) is formed. ; 20 May 1940– In France, British armoured units counter-attack at Arras. ; 26 May 1940 – The Dunkirk evacuation begins; over 330,000 British and French soldiers are evacuated by 4 June. ; 22 June 1940 – The Parachute Corps (later The Parachute Regiment) is formed. ; April 1941 – Germany invades Crete; Army and Commonwealth forces eventually 
Title: 1940 in the United Kingdom
Content:  Events from the year 1940 in the United Kingdom. The year was dominated by Britain's involvement in the Second World War, which commenced in September the previous year, as well as the numerous enemy air raids on Britain and thousands of subsequent casualties. Although the war continued, Britain did triumph in the Battle of Britain and foiled Nazi Germany's invasion attempt.
Title: Germany–United Kingdom relations
Content:  Germany and Britain fought each other from the British declaration of war, in September 1939, to the German surrender, in May 1945. The war continues to loom large in the British public memory. At the beginning of the war, Germany crushed Poland. In spring 1940, Germany astonished the world by quickly invading the Low Countries and France, driving the British army off the Continent and seizing most of its weapons, vehicles and supplies. War was brought to the British skies in the Battle of Britain in late summer 1940, but the aerial assault was repulsed, which stopped Operation Sealion, the plans for the invasion of Britain. The British Empire was standing alone against Germany, but the United States greatly funded and supplied the British. In December 1941, United States entered the war against Germany and Japan after the attack on Pearl Harbor by Japan, which also later overwhelmed British outposts in the Pacific from Hong Kong to Singapore. The Allied invasion of France on D-Day in June 1944 as well as strategic bombing and land forces all contributed to the final defeat of Germany.

Question: when did the united kingdom entered world war 2?
Answer: 1939.