Question: When did the UK enter WWI?

British entry into World War I

Great Britain entered World War I on 4 August 1914 when the king declared war after the expiration of an ultimatum to Germany.


Background



For much of the 19th century, Britain pursued a policy later known as splendid isolation, which sought to maintain the balance of power in Europe without formal alliances. As Europe divided into two power blocs during the 1890s, the 1895-1905 Conservative government realised this left Britain dangerously exposed.[1] This resulted in the 1902 Anglo-Japanese Alliance, followed by King Edward VII's 1903 visit to Paris. By reducing anti-British feeling in France, it led to the 1904 Entente Cordiale, the first tangible impact of which was British support for France against Germany in the 1905 Moroccan Crisis.
In 1907, the new Liberal government agreed the Anglo-Russian Convention. Like the Entente, the Convention focused on resolving colonial disputes but by doing so, paved the way for wider co-operation and allowed Britain to refocus its naval resources in response to German naval expansion.[2]

The 1911 Agadir Crisis encouraged secret military negotiations between France and Britain in the case of war with Germany. A British Expeditionary Force of 100,000 men would be landed in France within two weeks of war, while naval arrangements allocated responsibility for the Mediterranean to France, with the Royal Navy looking after the North Sea and the Channel, including Northern France.[3] Britain was effectively bound to support France in a war against Germany regardless but this was not widely understood outside government or the military.
Decision for war
British leaders increasingly had a sense of commitment to defending France against Germany. Firstly, if Germany again conquered France as had happened in the Franco-Prussian War, it would become a major threat to British economic, political and cultural interests. Secondly, partisanship was involved.  The Liberal Party was identified with internationalism and free trade,  and opposition to jingoism and warfare. By contrast the Conservative Party was identified as the party of nationalism and patriotism; Britons expected it "to show capacity in running a war." [4]  Liberal voters demanded peace, but they also were outraged when the Germans treated Belgian neutrality as a worthless "scrap of paper" (in the words of the German chancellor ridiculing the Treaty of London (1839)). Germany, en route to a massive attack on France, invaded Belgium early on the morning of 4 August. The victims called upon Britain for military rescue under the 1839 treaty and in response, London gave Berlin an ultimatum which expired at 11pm London time, which was ignored. The king declared war on Germany that same evening August 4, 1914.[5]
As late as August 1, 1914, the great majority of Liberal—both voters and cabinet members—strongly opposed going to war.[6] The German invasion of Belgium was such an outrageous violation of international rights that Liberals agreed for war on August 4.  Historian Zara Steiner says: 
The public mood did change. Belgium proved to be a catalyst which unleashed the many emotions, rationalizations, and glorifications of war which had long been part of the British climate of opinion. Having a moral cause, all the latent anti-German feelings, that by years of naval rivalry and assumed enmity, rose to the surface. The 'scrap of paper' proved decisive both in maintaining the unity of the government and then in providing a focal point for public feeling.[7]
The Liberals succeeded in mending their deep division over warfare. Unless the Liberal government acted decisively against the German invasion of France, its top leaders including Prime Minister  H.H. Asquith, Foreign Minister Edward Grey, First Lord of the Admiralty (Navy Minister) Winston Churchill and others would resign, leading to a risk that the much more pro-war Conservative Party might form a government. Mistreatment of Belgium was not itself a fundamental cause of British entry, but it was used extensively as a justification in wartime propaganda to motivate the British people.[8]
The German high command was aware that entering Belgium would trigger British intervention but decided the risk was acceptable; they expected it to be a short war while their ambassador in London claimed civil war in Ireland would prevent Britain from assisting France.[9]
Historians looking at the July crisis typically conclude that Grey:
was not a great foreign secretary but an honest, reticent, punctilious English gentleman.... He exhibited a judicious understanding of European affairs, a firm control of his staff, and a suppleness and tact in diplomacy, but he had no boldness, no imagination, no ability to command men and events. [Regarding the war] He pursued a cautious, moderate policy, one that not only fitted his temperament, but also reflected the deep split in the Cabinet, in the Liberal party, and in public opinion.[10]

Irish crisis on hold
Until late July, British politics was totally focused on the threat of civil war in Ireland. In 1912 the government had presented a Home Rule bill that Irish nationalists demanded; under the terms of the Parliament Act 1911, by which the House of Lords retained the right to delay legislation by up to two years, it was due to become law in 1914. The Ulster Protestants demanded separate treatment; by 1914 the government was offering a six-year opt-out to the six counties which would eventually become Northern Ireland, but not the permanent exemption which they demanded. Both sides in Ireland had smuggled in weapons, set up militias with tens of thousands of volunteers, were drilling, and were ready to fight a civil war. The British Army itself was paralyzed: during the Curragh Incident officers threatened to resign or accept dismissal rather than obey orders to deploy into Ulster. Elements of the Unionist (Conservative) Party supported them. Suddenly on July 25 the Austrian ultimatum to Serbia became known, and the cabinet realized that war with Germany was increasingly likely. The Government of Ireland Act 1914 was enacted into law, but suspended for the duration of hostilities, with the issue of Ulster still unresolved.[11]  Grey told Parliament on August 3:The one bright spot in the whole of this terrible situation is Ireland. [Prolonged cheers.] The general feeling throughout Ireland, and I would like this to be clearly understood abroad, does not make that a consideration that we feel we have to take into account.[Cheers.][12]
Propaganda
Before war was declared, newspapers gave extensive coverage but ranged widely in recommended policy recommendations.[13][14]
Once war was declared defense of Belgium rather than France was the public reason given for war.  Propaganda posters emphasized that Britain was required to safeguard Belgium's neutrality under the 1839 Treaty of London.[15][16]

Propaganda posters emphasized that Britain was required to safeguard Belgium's neutrality under the 1839 Treaty of London.
Empire at war
The declaration of war automatically involved all dominions and colonies and protectorates of the British Empire, many of whom made significant contributions to the Allied war effort, both in the provision of troops and civilian labourers.
See also
Causes of World War I
Austro-Hungarian entry into World War I
French entry into World War I
German entry into World War I
Ottoman entry into World War I
Russian entry into World War I
Allies of World War I
France–United Kingdom relations
Germany–United Kingdom relations
British military history
History of the United Kingdom, since 1707
International relations of the Great Powers (1814–1919)
Diplomatic history of World War I
International relations (1919–1939)
Timeline of British diplomatic history
History of the foreign relations of the United Kingdom
Notes

Further reading

Anderson, Frank Maloy, and Amos Shartle Hershey, eds. Handbook For The Diplomatic History Of Europe, Asia, and Africa, 1870-1914 (1918) 
Bartlett, Christopher John. Defence and diplomacy: Britain and the Great Powers, 1815-1914 (Manchester UP, 1993).
Bartlett, C. J. British Foreign Policy in the Twentieth Century (1989).
Brandenburg, Erich. (1927) From Bismarck to the World War: A History of German Foreign Policy 1870–1914 (1927) .
Bridge F. R. Great Britain and Austria-Hungary 1906-14 (1972).
Charmley, John. Splendid Isolation?: Britain, the Balance of Power and the Origins of the First World War (1999), highly critical of Grey.
Clark, Christopher. The Sleepwalkers: How Europe Went to War in 1914 (2013) 
Sleepwalkers lecture by Clark. 
Ensor, R. C. K.  England, 1870–1914 (1936) 
 essays by scholars from both sides
Fay, Sidney B. The Origins of the World War (2 vols in one. 2nd ed. 1930). , passim
French, David. British Economic and Strategic Planning 1905-15 (1982).
Goodlad, Graham D. British Foreign and Imperial Policy 1865–1919 (1999).
Hale, Oron James. Publicity and Diplomacy: With Special Reference to England and Germany, 1890-1914 (1940) 
Hamilton, Richard F. and Holger H. Herwig, eds. War Planning 1914 (2014) pp 48–79
Hamilton, Richard F. and Holger H. Herwig, eds. The Origins of World War I (2003) pp 266–299.
Hamilton, Richard F.. and Holger H. Herwig. Decisions for War, 1914-1917 (2004).
Hinsley, F. H. ed. British Foreigh Policy under Sir Edward Grey (1977) 

Kennedy, Paul. The Rise of the Anglo-German Antagonism 1860–1914 (1980) in-depth Coverage of diplomacy, military planning,  business and cultural relationships, propaganda and public opinion 
Kennedy, Paul. The Rise and Fall of the Great Powers (1987), pp 194–260. 
Kennedy, Paul. The Rise and Fall of British Naval mastery (1976) pp 205–38.
Kennedy, Paul M. "Idealists and realists: British views of Germany, 1864–1939." Transactions of the Royal Historical Society 25 (1975): 137-156.  
McMeekin, Sean. July 1914: Countdown to War (2014) scholarly account, day-by-day
; major scholarly overview
Matzke, Rebecca Berens. . Deterrence through Strength: British Naval Power and Foreign Policy under Pax Britannica (2011) 
Mowat, R. B. "Great Britain and Germany in the Early Twentieth Century" English Historical Review (1931) 46#183 pp.423–441 
Murray, Michelle. "Identity, insecurity, and great power politics: the tragedy of German naval ambition before the First World War." Security Studies 19.4 (2010): 656-688. 
Neilson,  Keith. Britain and the Last Tsar: British Policy and Russia, 1894-1917 (1995) 
Otte, T. G. July Crisis: The World's Descent into War, Summer 1914 (Cambridge UP, 2014). 
Paddock, Troy R. E. A Call to Arms: Propaganda, Public Opinion, and Newspapers in the Great War  (2004) 
Padfield, Peter. The Great Naval Race: Anglo-German Naval Rivalry 1900-1914 (2005)
Papayoanou, Paul A. "Interdependence, institutions, and the balance of power: Britain, Germany, and World War I." International Security 20.4 (1996): 42-76.
Rich, Norman. Great Power Diplomacy: 1814-1914 (1991), comprehensive survey
Ritter, Gerhard. The Sword and the Sceptre, Vol. 2-The European Powers and the Wilhelmenian Empire 1890-1914  (1970) Covers military policy in Germany and also France, Britain, Russia and Austria.
Schmitt, Bernadotte E. "Triple Alliance and Triple Entente, 1902-1914." American Historical Review 29.3 (1924): 449-473. 
Schmitt, Bernadotte Everly.  England and Germany, 1740-1914 (1916). 
Scott, Jonathan French. Five Weeks: The Surge of Public Opinion on the Eve of the Great War (1927)   pp 99–153 .
Seton-Watson, R. W. Britain in Europe, 1789–1914, a survey of foreign policy (1937) useful overview 
Steiner, Zara S. Britain and the origins of the First World War (1977), a major scholarly survey.
Stowell, Ellery Cory. The Diplomacy of the War of 1914 (1915)  728 pages 

Tucker, Spencer C., ed. The European Powers in the First World War: An Encyclopedia  (1996) 816pp.
Vyvyan, J. M. K. "The Approach of the War of 1914." in C. L. Mowat, ed. The New Cambridge Modern History: Vol. XII: The Shifting Balance of World Forces 1898-1945 (2nd ed. 1968)  pp 140–70.
Ward A.W., ed. The Cambridge History Of British Foreign Policy 1783-1919 Vol III  1866-1919 (1923) 
Williamson Jr., Samuel R. "German Perceptions of the Triple Entente after 1911: Their Mounting Apprehensions Reconsidered" Foreign Policy Analysis 7.2 (2011): 205-214.
Williamson, Samuel R. The politics of grand strategy: Britain and France prepare for war, 1904-1914 (1990).
Wilson, Keith M. "The British Cabinet's decision for war, 2 August 1914." Review of International Studies 1.2 (1975): 148-159.
Wood, Harry. "Sharpening the Mind: The German Menace and Edwardian National Identity." Edwardian Culture (2017). 115-132. public fears of German invasion.
Woodward, E.L. Great Britain And The German Navy (1935) 535pp; scholarly history 
Young, John W. "Ambassador George Buchanan and the July Crisis." International History Review 40.1 (2018): 206-224.
Historiography
Herwig,  Holger H. ed., The Outbreak of World War I: Causes and Responsibilities (1990) excerpts from primary and secondary sources
Horne, John, ed. A Companion to World War I (2012) 38 topics essays by scholars
Kramer, Alan. "Recent Historiography of the First World War – Part I", Journal of Modern European History (Feb. 2014) 12#1 pp 5–27; "Recent Historiography of the First World War (Part II)", (May 2014) 12#2 pp 155–174.
Langdon, John W. "Emerging from Fischer's shadow: recent examinations of the crisis of July 1914." History Teacher 20.1 (1986): 63-86, historiography 
Mombauer, Annika. "Guilt or Responsibility? The Hundred-Year Debate on the Origins of World War I." Central European History 48.4 (2015): 541-564.
Mulligan, William. "The Trial Continues: New Directions in the Study of the Origins of the First World War." English Historical Review (2014) 129#538 pp: 639–666.
Winter, Jay. and Antoine Prost eds.  The Great War in History: Debates and Controversies, 1914 to the Present (2005)
Primary sources
Albertini, Luigi. The Origins of the War of 1914 (3 vol 1952).
Barker. Ernest, et al. eds. Why we are at war; Great Britain's case (3rd ed. 1914), the official British case against Germany. 
Gooch, G.P. Recent revelations of European diplomacy (1928) pp 3-101. 

Gooch, G.P. and Harold Temperley, eds. British documents on the origins of the war, 1898-1914 (11 vol.) 
v. i The end of British isolation -- v.2. The Anglo-Japanese Alliance and the Franco-British Entente -- v.3. The testing of the Entente, 1904-6 -- v.4. The Anglo-Russian rapprochment, 1903-7 -- v.5. The Near East, 1903-9 -- v.6. Anglo-German tension. Armaments and negotiation, 1907-12 -- v.7. The Agadir crisis -- v.8. Arbitration, neutrality and security -- v.9. The Balkan wars, pt.1-2 -- v.10, pt.1. The Near and Middle East on the eve of war. pt.2. The last years of peace -- v.11. The outbreak of war V.3. The testing of the Entente, 1904-6 -- v.4. The Anglo-Russian rapprochment, 1903-7 -- v.5. The Near East, 1903-9 -- v.6. Anglo-German tension. Armaments and negotiation, 1907-12 -- v.7. The Agadir crisis -- v.8. Arbitration, neutrality and security -- v.9. The Balkan wars, pt.1-2 -- v.10, pt.1. The Near and Middle East on the eve of war. pt.2. The last years of peace --   v.11. The outbreak of war.
Joll, James, ed. Britain and Europe 1793-1940 (1967); 390pp of documents
Jones, Edgar Rees, ed. Selected speeches on British foreign policy, 1738-1914 (1914). 
Lowe, C.J. and Michael L. Dockrill, eds. Mirage of Power: The Documents v. 3: British Foreign Policy (1972); vol 3 = primary sources 1902-1922
Scott, James Brown, ed., Diplomatic Documents Relating To The Outbreak Of The European War (1916) 
Wilson, K.M. "The British Cabinet's Decision for War, 2 August 1914" British Journal of International Studies 1#3 (1975), pp.148–159