From War on the Rocks:

The British Army suffered a bloody and bruising experience in the Boer War. The war erupted in South Africa in October 1899 when the Boer republics of Orange Free State and Transvaal, infuriated by years of British political pressure and military threats, launched a pre-emptive war to secure their independence. Most of the British press greeted this with jingoistic glee and some journalists confidently predicted that the war would be “over by Christmas.”

But the war confounded expectations. Much of this was down to the nature of the Boer military. Although the Boers lacked a standing army, they possessed an effective militia system that mustered white citizens into units based on their local district. These men had no formal training but came from a frontier culture that produced tough individuals, many of whom were skilled marksmen and experienced riders. Most Boers went to war with their own rifles and horses, and the government provided equipment and mounts if required. These rugged frontiersmen would prove hardy and highly mobile fighters. It is little surprise that Winston Churchill, a veteran of the war himself, would later choose to name Britain’s first special forces Commandos, taking the word from the Boer term for military units.