When London asked whether South Africa would invade German South West Africa, Botha answered yes. It was not a simple yes. Barely a dozen years had passed since the Second Boer War, when Britain had crushed the two Boer republics and Germany had quietly cheered the Boer cause. Many Afrikaners now being asked to fight for the British Crown remembered exactly whose side Germany had been on. The wound was fresh, and Botha — himself a former Boer commander — was asking old comrades to march under the flag of their recent enemy. Some refused outright. The question of which Africa this would be, and whose empire it would serve, was settled not on the German border but inside South Africa itself.

Lieutenant-Colonel Manie Maritz, commanding troops on the very border he was meant to cross, turned instead and joined the Germans. Around twelve thousand men rose with him across the Transvaal and Orange Free State in what became known as the Maritz rebellion. Botha declared martial law on 14 October 1914 and went after them with Jan Smuts at his side — Boer generals hunting Boer rebels across the same grassland where they had once fought together against Britain. Maritz was beaten on 24 October and fled across the border. By February the revolt was broken. The ringleaders drew six and seven years in prison, but within two years Botha had them released. He understood, perhaps better than anyone, that a country could not be governed on grudges.

With his home front secured, Botha turned outward. He landed at the coastal town of Swakopmund in February 1915 and pushed inland up the Swakop valley, following the railway line through Otjimbingwe, Karibib, and Okahandja. On 5 May his northern columns entered Windhoek, the German capital, and a week later the city was formally occupied. Botha then split the colony in two and sent his columns fanning northeast in pursuit. The men leading them had ridden in Boer commandos, and they moved with the speed of horsemen who knew how to live off hard country. Far to the south, Smuts landed at Lüderitz and drove inland, taking Keetmanshoop and then Gibeon after two days of fighting, squeezing the Germans between his advance and Botha's.

The German forces made their last stand at the Battle of Otavi on 1 July 1915. Eight days later, on 9 July, Governor Theodor Seitz and his commander surrendered at Khorab, north of Windhoek. When the Germans handed over lists naming roughly 2,200 troops, Botha accused them of trickery — he was certain they had fielded thousands more. Victor Franke, the German commander, gave a reply that has outlived the campaign: "If we had 15,000 men then you wouldn't be here and we wouldn't be in this position." The cost had been remarkably low. South Africa lost 113 men killed, with another 153 dead of injury or illness; the Germans lost 103 killed and 890 taken prisoner, along with 37 field guns. It was one of the first clear Allied victories of the entire war.

Victory in the field opened a chapter that would last three-quarters of a century. The colony being handed over was already scarred by atrocity: between 1904 and 1908, the German Schutztruppe under General Lothar von Trotha had carried out a systematic genocide against the Herero people and the Nama people — Germany officially recognized these events as genocide in 2021. South Africa now governed that same territory, first as a League of Nations mandate from 1919, then as a de facto fifth province where only the white minority held a vote. When the United Nations replaced the League in 1946, South Africa refused to give up the territory, and in 1971 the International Court of Justice ruled its administration illegal. The fast cavalry campaign of 1915 had been the easy part. The land it delivered would not become an independent Namibia until 1990 — a long shadow cast by a short war, on ground that had already seen the worst.