The end of the Britsh Empire began on May 31, 1902.
On that day, the Treaty of Vereeniging ended the three-year long disaster known as the Boer War. It began as a dispute over mining rights and sovereignty of the Boer Republics of South Africa, the Transvaal and the Orange Free State. It ended as one of the darkest chapters in British history.
The war was technically “over” in 1900, when the British occupied the Transvaal capital of Pretoria. However, the remaining Boer commandos of the Orange Free State and the Northern Transvaal continued a war of attrition for another two years. It would see unspeakable atrocities on both sides. It would see “scorched-Earth” tactics and concentration camps that would result in the deaths of thousands. It would also see continued and violent repression, mutilation and torture of the majority African native population–a situation not really rectified until almost a century later.
Finally, the Boer War would see British people start to question the need for a colonial empire. Though a victory, the war cost thousands of lives and millions of British pounds. Britons would then start questioning the use of British troops, the entanglement in colonial affairs–even questioning the need for an empire in the first place.
Attached is a nice 5-part synopsis of the Boer War and other African conflicts of the time. It is very even handed, and its short length is perfect for the classroom.