The outbreak of war with Germany in 1914 finds the party split, with many leading figures in the movement fearing the war will hit the poor hardest. But the majority of the party soon swings behind the war effort following outrage sparked by Germany's invasion of Belgium, and trade unions benefit from the increased demand for labour.
The party picks up its first ministerial posts in the wartime national government, gaining valuable experience of office.
Shortly before the return to peace in 1918 Labour puts in place a new constitution, drafted by Sidney Webb. It gives fresh life to Labour's pitch to the voters and enshrines a commitment to socialism in Clause IV.
In the 1918 election Labour increases its number of MPs to 57. Liberal fortunes continue to plummet and at the 1922 election Labour replaces it as the UK's second largest party, becoming the official opposition for the first time.