1. France: France was Britain's closest ally during the war. The two countries had a formal alliance since 1904, and they fought side by side on the Western Front. France provided Britain with millions of soldiers, as well as supplies and munitions.
2. Russia: Russia was another major ally of Britain. The two countries had an informal alliance since 1907, and they fought together on the Eastern Front. Russia provided Britain with millions of soldiers, but they also tied up large numbers of German troops, which helped to ease the pressure on the Western Front.
3. United States: The United States entered the war in 1917, after Germany resumed unrestricted submarine warfare. The United States provided Britain with millions of troops, as well as supplies and munitions. The United States also played a major role in the naval war, helping to protect British shipping lanes.
4. Italy: Italy entered the war in 1915, after signing the Treaty of London with Britain and France. Italy provided Britain with troops, as well as supplies and munitions. The Italian army fought on the Italian Front, which helped to tie up large numbers of Austrian troops.
5. Japan: Japan entered the war in 1914, after declaring war on Germany. Japan provided Britain with troops, as well as supplies and munitions. The Japanese navy also played a role in the naval war, helping to protect British shipping lanes.
6. Canada: Canada was a self-governing dominion of the British Empire, and it provided Britain with millions of troops, as well as supplies and munitions. The Canadian army fought on the Western Front, and it played a major role in several major battles, including the Battle of Vimy Ridge.
7. Australia: Australia was another self-governing dominion of the British Empire, and it provided Britain with troops, as well as supplies and munitions. The Australian army fought on the Western Front, and it also played a role in the Gallipoli Campaign.
8. New Zealand: New Zealand was a self-governing dominion of the British Empire, and it provided Britain with troops, as well as supplies and munitions. The New Zealand army fought on the Western Front, and it also played a role in the Gallipoli Campaign.
9. India: India was a British colony, and it provided Britain with millions of troops, as well as supplies and munitions. The Indian army fought on the Western Front, as well as in the Middle East and Africa.
10. South Africa: South Africa was a self-governing dominion of the British Empire, and it provided Britain with troops, as well as supplies and munitions. The South African army fought on the Western Front, as well as in the Middle East and Africa.