At the end of July, 1914, it became clear to the British government that the country was on the verge of war with Germany. Four senior members of the government, David Lloyd George (Chancellor of the Exchequer), Charles Trevelyan (Parliamentary Secretary of the Board of Education), John Burns (President of the Local Government Board) and John Morley (Secretary of State for India), were opposed to the country becoming involved in a European war. They informed the Prime Minister, Herbert Asquith, that they intended to resign over the issue. When war was declared on 4th August, three of the men, Trevelyan, Burns and Morley, resigned, but Asquith managed to persuade Lloyd George, his Chancellor of the Exchequer, to change his mind.

The day after war was declared, Trevelyan began contacting friends about a new political organisation he intended to form to oppose the war. This included two pacifist members of the Liberal Party, Norman Angell and E. D. Morel, and Ramsay MacDonald, the leader of the Labour Party. A meeting was held and after considering names such as the Peoples' Emancipation Committee and the Peoples' Freedom League, they selected the Union of Democratic Control.

The four men agreed that one of the main reasons for the conflict was the secret diplomacy of people like Britain's foreign secretary, Sir Edward Grey. They decided that the Union of Democratic Control should have three main objectives: (1) that in future to prevent secret diplomacy there should be parliamentary control over foreign policy; (2) there should be negotiations after the war with other democratic European countries in an attempt to form an organisation to help prevent future conflicts; (3) that at the end of the war the peace terms should neither humiliate the defeated nation nor artificially rearrange frontiers as this might provide a cause for future wars.

The founders of the Union of Democratic Control produced a manifesto and invited people to support it. Over the next few weeks several leading figures joined the organisation. This included members of the Liberal Party (Arthur Ponsonby, J. A. Hobson, Charles Buxton, Frederick Pethick-Lawrence, Norman Angell, Arnold Rowntree, Philip Morrel, Morgan Philips Price, George Cadbury) and the Labour Party (Helena Swanwick, Fred Jowett, Ramsay MacDonald, Tom Johnston, Philip Snowden, Arthur Henderson, David Kirkwood, William Anderson, Isabella Ford, H. H. Brailsford). Others who joined who were not actively involved in party politics at that time included Israel Zangwill, Bertrand Russell, Margaret Llewelyn Davies, Margaret Sackville and Morgan Philips Price).

Trevelyan's house (14 Great College Street, London) became the UDC's headquarters. As the organisation expanded the organisation took larger premises at 37 Norfolk Street (1915) and 4-7 Lion Court, Fleet Street (1917). The UDC was mainly funded by prosperous Quaker businessmen such as George Cadbury and Arnold Rowntree.

The UDC was one of the first political groups to appoint women to senior positions in an organisation. Helena Swanwick was a member of the Executive Committee and twelve women were on the General Council. This included Isabella Ford, Margaret Llewelyn Davies and Margaret Sackville.

The Union of Democratic Control soon emerged at the most important of all the anti-war organizations in Britain and by 1915 had 300,000 members. E. D. Morel, as secretary and treasurer, became the dominant figure in the UDC. In August 1915, the UDC decided to pay Morel for his secretarial duties. Morel also wrote most of the UDC pamphlets published during the war. Others who wrote pamphlets included Ramsay MacDonald, Norman Angell, Arthur Ponsonby, J. A. Hobson, Charles Buxton, Norman Angell, Helena Swanwick, Richard Tawney and H. H. Brailsford. Members of the UDC also established a League of Nations Society.

Whereas the Manchester Guardian and The Nation, were fairly sympathetic to the aims of the UDC, most newspapers and journals were extremely hostile. The Daily Express, edited by Ralph Blumenfeld, led the campaign against the UDC. In April 1915 it printed wanted posters of E. D. Morel, Ramsay MacDonald and Norman Angell. Under headings such as: 'Who is E. D. Morel? And Who Pays for his Pro-German Union? it suggested that the UDC was working for the German government. The Daily Express also listed details of future UDC meetings and encouraged its readers to go and break-up them up.

Although the UDC complained to the Home Secretary about what it called "an incitement to violence" by the Daily Express, he refused to take any action. Over the next few months the police refuse to protect UDC speakers and they were often attacked by angry crowds. After one particularly violent event on 29th November, 1915, the Daily Express proudly reported the "utter rout of the pro-Germans".

The Daily Sketch joined the campaign against the UDC. It told its readers on 1st December, 1915, that to: "kill this conspiracy we must get hold of the arch-conspirator, E. D. Morel". Over the next few months E. D. Morel was physically attacked several times. He continued to run the organisation and by 1917 membership of the UDC and affiliated organizations had reached 650,000.

The government now saw E. D. Morel as an extremely dangerous political figure. Basil Thompson, head of the Criminal Investigation Division of Scotland Yard, and future head of Special Branch, was asked to investigate Morel and the Union of Democratic Control. Thompson reported that the UDC was not a revolutionary body and its funds came from the Society of Friends and "Messrs. Cadbury, Fry and Rowntree".

Despite Thompson's failure to find any evidence of criminal activity, the Home Secretary gave instructions for Morel's arrest. On the 22nd August, 1917 Morel's house was searched and evidence was discovered that he had sent a UDC pamphlet to a friend living in Switzerland. This was a technical violation of the the Defence of the Realm Act and Morel was sentenced to six months in prison. Morel, whose health was already poor, never fully recovered from the harsh conditions of Pentonville Prison.

In the 1918 General Election all the leading members of the Union of Democratic Control lost their seats in Parliament. However, by 1924, they had returned and several, including Ramsay MacDonald (Prime Minister/Foreign Secretary), Philip Snowden (Chancellor of the Exchequer), Arthur Henderson (Home Secretary), Charles Trevelyan (Minister of Education) and Fred Jowett (Commissioner of Works) were all members of the new Labour Government. E. D. Morel was not given a Cabinet post but was MacDonald's leading adviser at the Foreign Office.

Members of the Union Democratic Control were strong opponents of the Versailles Treaty. Several senior army officers joined the UDC in protest against the treaty including General Hubert Gough, Brigadier-General C. B. Thompson, Commander Kenworthy and Colonel Bruce Kingsmill.