After years of escalation — which included a war and a British-led raid to take Johannesburg — the Boer republics of Transvaal and the Orange Free State invade the British-held Cape Colony and Natal, beginning the Second Boer War. Talk about being united and divided simultaneously, eh?
The war began with rapid invasions by the Boers, who had the great advantage of having a fighting force that was nimble, familiar with the area, good on horses and fairly easy to mobilize. Within a few weeks, the Boers had made great inroads into British-held territory, driving back the British in some cases and laying siege to garrisons in others.
Spoiler alert: though the war pioneered the use of modern guerrilla warfare, the Brits ultimately win this one, the Boer territories are incorporated into the British colonies and the Union of South Africa comes to be in 1910…and then develops into a truly reprehensible country but then gets sort of OK again and then hosts the World Cup.