What sparked the Boer War?
English annexation of South Africa and the revolt of the Dutch farmers ("Boers") living there.
South Africa started being colonized in the 1650s by the Dutch. While there were skirmishes and fighting with the local peoples, there was also trading and diplomatic relations. Dutch settlers began claiming land and building towns and cities.
Meanwhile, the British spent a lot of time trying to take South Africa away from the Dutch. The British managed to take little bits and pieces (which it would then give back) but never managed to take all of South Africa for itself.
(And while all of this was going on, the Zulus were building an empire within South Africa and challenging the foreign intruders, which was also leading to war and bloodshed).
In 1879, the English sent in an army to fight the Zulus and they lost initially. Badly. Embarrassingly. And so the British sent the overwhelming might of the army to quell the Zulus and eventually, they did.
With the Zulus defeated, the British then annexed South Africa and imposed English rule on the inhabitants. Which included the Dutch settlers (who were known as "Boers", which translates to "farmers"). The Boers weren't happy with British rule and so revolted in 1880 - which marked the start of the First Boer War. The Boers defeated the British, who sued for peace in 1881. (But it all started up again 18 years later with the Second Boer War which the Boers lost).
So why was England interested in South Africa at all? (Thanks to @Lucio Margherita for the heads up). Three things:
The Napoleonic Wars (with England at war with France, and France having effectively annexed the Netherlands, Britain saw an opportunity to defeat the Dutch/French),
The dream of empire (people like Cecil Rhodes, who arguably was a white supremacist, viewed the world as a place that should be ruled by white people - and since he happened to be English - English white people. His plan was to capture the east side of Africa and unite it through the creation of a railway from Egypt to South Africa.
The French, Portuguese , and Germans all had similar plans to link their colonies in Africa using a rail line. None succeeded, but the English got closest (at one point they had colonized all the territory they needed - the result of taking over territory from German in WW2) but the resources were never there to finish the project. And,
- Wealth. South Africa is home to extensive gold and diamond deposits (in fact, Cecil Rhodes is the father of De Beers diamond mines and he used that wealth to drive his expansionist plans).
The Boer War, then, was the culmination of war in far off places, expansionism, and greed.