On June 28, 1914, Gavrilo Princip, a Serbian nationalist, assassinated Archduke Franz Ferdinand of the Austro-Hungarian Empire. Austria-Hungary immediately issued a harsh ultimatum, and when Serbia failed to reply, they declared war. Within a week, Germany joined them. Russia, France, and the United Kingdom formed the Triple Entente in response.

Without a large standing army, the United Kingdom called for volunteers from the British Isles and colonies, such as Australia, Canada, and New Zealand among others. These volunteers, often referred to as Kitchener’s Army, fought on land, and sea and in the air. They were also the first to arrive in France to combat the German Army that had invaded through Belgium and began a destructive assault on Liege as part of the Schlieffen Plan to capture France before its allies could arrive.

The first major battle of WWI occurred on September 6, 1914, at the Marne, where French and British troops confronted the invading Germans. They halted the German advance and mounted a successful counteroffensive, pushing the Germans back and stopping their progression toward Paris. After this victory, the Western Front settled into a stalemate, and both sides dug trenches and settled in for a long war of attrition.

In the East, the Allies mounted an attack on the Ottoman Empire, which had joined the Triple Alliance at the end of 1914. The Allies first attempted an amphibious assault to the Dardanelles Straits, but they were unsuccessful. Unable to overpower the Ottoman defenses, they switched from a primarily naval confrontation to a land-based assault backed by naval forces. In April 1915, the Allies launched a land invasion of the Gallipoli Peninsula, a small stretch of land between the Aegean Sea and the Sea of Marmara. The invasion was unsuccessful, and the Allies retreated in January 1916.

In that same month, the UK introduced conscription for the first time in its history. Men between the ages of 18 and 41 were conscripted into the military from England, Wales, and Scotland. As more men left for the war front, many women began to take industrial positions in factories, and others joined the war effort as nurses, medical women, and even navy, air force, or army as servicewomen.

The UK would need the additional troops as 1916 saw them fight in both Jutland and the Somme. Jutland was the biggest naval battle of the war with 250 ships and about 100,000 troops involved. After codebreakers cracked the German codes, Admiral Sir John Jellicoe set sail to fend off a secret German advance. Both sides incurred heavy losses, but the UK came out victorious and maintained control of the North Sea. Just a few months later, Allied forces attacked German trenches at the Somme, where they used tanks for the first time in history. Though they hoped to overwhelm German forces quickly, they were unsuccessful, lost tens of thousands of men, and settled back into a war of attrition.

Despite a growing army, the UK still struggled to combat German attacks, especially those by submarine. At the beginning of the war, Germany had indiscriminately attacked merchant and passenger ships but later agreed to cease attacks on unarmed passenger ships after the tragic sinking of the Lusitania. This peace did not last though, and in February 1917, Germany declared unrestricted submarine warfare and established the waters around the British Isles as war zones. Within two months of this announcement, the US declared war on Germany and joined the Allies.

Hoping to capture France before American reinforcements arrived, Germany launched its Spring Offensive. Small infantry groups and brief artillery fire allowed German forces to push into France and get within 75 miles of Paris; however, American reinforcements soon arrived, and the Allies resisted the German invasion. They then launched the Hundred Days Offensive, a series of coordinated attacks across the Western Front. Outnumbered and overwhelmed, Germany agreed to an armistice on November 11, 1918. Seven months later, Germany agreed to the Treaty of Versailles, and the First World War formally ended.

Over the course of the war, the United Kingdom and its colonies suffered over 3 million casualties. Veterans received military medals, Distinguished Conduct medals, and pensions for their service. More information on their experiences can be found in the Stories of Heroism, diaries from the Eastern and Western Fronts, biographies of British and Irish officers, and the British Jewry Book of Honour.

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