The trade union and socialist movements had long voiced their opposition to a war, which they argued, meant only that workers would kill other workers in the interest of capitalism. Once war was declared, however, many socialists and trade unions backed their governments. Among the exceptions were the Bolsheviks, the Socialist Party of America, and the Italian Socialist Party, and individuals such as Karl Liebknecht, Rosa Luxemburg and their followers in Germany. There were also small anti-war groups in Britain and France.
In Britain, in 1914, the Public Schools Officers' Training Corps annual camp was held at Tidworth Pennings, near Salisbury Plain. Head of the British Army Lord Kitchener was to review the cadets, but the immenence of the war prevented him. General Horace Smith-Dorrien was sent instead. He surprised the two-or-three thousand cadets by declaring (in the words of Donald Christopher Smith, a Bermudian cadet who was present) that war should be avoided at almost any cost, that war would solve nothing, that the whole of Europe and more besides would be reduced to ruin, and that the loss of life would be so large that whole populations would be decimated. In our ignorance I, and many of us, felt almost ashamed of a British General who uttered such depressing and unpatriotic sentiments, but during the next four years, those of us who survived the holocaust-probably not more than one-quarter of us - learned how right the General's prognosis was and how courageous he had been to utter it. Having voiced these sentiments did not hinder Smith-Dorien's career, or preventing him from doing his duty in the First World War to the best of his abilities.
Many countries jailed those who spoke out against the conflict. These included Eugene Debs in the United States and Bertrand Russell in Britain. In the U.S., theEspionage Act of 1917 effectively made free speech illegal and many served long prison sentences for statements of fact deemed unpatriotic. The Sedition Act of 1918made any statements deemed "disloyal" a federal crime. Publications at all critical of the government were removed from circulation by postal censors.
A number of nationalists opposed intervention, particularly within states that the nationalists held hostility to. Irish nationalists staunchly opposed taking part in intervention with the United Kingdom of Great Britain and Ireland. The war had begun amid the Home Rule crisis in Ireland that had begun in 1912 and by 1914 there was a serious possibility of an outbreak of civil war in Ireland between Irish unionists and republicans. Irish nationalists and Marxists attempted to pursue Irish independence, culminating in the Easter Rising of 1916, with Germany sending 20,000 rifles to Ireland in order to stir unrest in the United Kingdom. The UK government placed Ireland under martial law in response to the Easter Rising.
Other opposition came from conscientious objectors – some socialist, some religious – who refused to fight. In Britain 16,000 people asked for conscientious objector status. Many suffered years of prison, including solitary confinement and bread and water diets. Even after the war, in Britain many job advertisements were marked "No conscientious objectors need apply".
The Central Asian Revolt started in the summer of 1916, when the Russian Empire government ended its exemption of Muslims from military service.
In 1917, a series of mutinies in the French army led to dozens of soldiers being executed and many more imprisoned.
In Milan in May 1917, Bolshevik revolutionaries organised and engaged in rioting calling for an end to the war, and managed to close down factories and stop public transportation. The Italian army was forced to enter Milan with tanks and machine guns to face Bolsheviks and anarchists who fought violently until May 23 when the army gained control of the city with almost fifty people killed (three of which were Italian soldiers) and over 800 people arrested.
The Conscription Crisis of 1917 in Canada erupted when conservative Prime Minister Robert Borden brought in compulsory military service over the objection of French-speaking Quebecers. Out of approximately 625,000 Canadians who served, about 60,000 were killed and another 173,000 were wounded.
In 1917, Emperor Charles I of Austria secretly entered into peace negotiations with the Allied powers, with his brother-in-law Sixtus as intermediary, without the knowledge of his ally Germany. He failed, however, because of the resistance of Italy.
In September 1917, the Russian soldiers in France began questioning why they were fighting for the French at all and mutinied. In Russia, opposition to the war led to soldiers also establishing their own revolutionary committees and helped foment the October Revolution of 1917, with the call going up for "bread, land, and peace". The Bolsheviks reached a peace treaty with Germany, the peace of Brest-Litovsk, despite its harsh conditions.
The end of October 1918, in northern Germany, saw the beginning of the German Revolution of 1918–19. Units of the German Navy refused to set sail for a last, large-scale operation in a war which they saw as good as lost, initiating the uprising. The sailors' revolt which then ensued in the naval ports of Wilhelmshaven and Kiel spread across the whole country within days and led to the proclamation of a republic on 9 November 1918 and shortly thereafter to the abdication of Kaiser Wilhelm II.
As the war slowly turned into a war of attrition, conscription was implemented in some countries. This issue was particularly explosive in Canada and Australia. In the former it opened a political gap between French-Canadians, who claimed their true loyalty was to Canada and not the British Empire, and the Anglophone majority who saw the war as a duty to both Britain and Canada. Prime Minister Robert Borden pushed through a Military Service Act, provoking the Conscription Crisis of 1917. In Australia, a sustained pro-conscription campaign by Prime Minister Billy Hughes, caused a split in the Australian Labor Party and Hughes formed the Nationalist Party of Australia in 1917 to pursue the matter. Nevertheless, the labour movement, the Catholic Church, and Irish nationalist expatriates successfully opposed Hughes' push, which was rejected in two plebiscites.
Conscription put into uniform nearly every physically fit man in Britain, six of ten million eligible. Of these, about 750,000 lost their lives and 1,700,000 were wounded. Most deaths were to young unmarried men; however, 160,000 wives lost husbands and 300,000 children lost fathers.