The Cape switched between Dutch and British hands during the Napoleonic Wars until 1815, when British occupation was confirmed by the Congress of Vienna. The already pronounced unruliness and wanderlust of the Dutch colonists was aggravated by the outlawing of the Dutch language (1806), and abolition of slavery (1838), on which their agricultural economy was based. This prompted the mass emigration of the Great Trek from the 1830s, which would ultimately result in the foundation of the independent Boer Republics of Transvaal (1852) and Orange Free State (1854). Meanwhile, the British colony developed a second hub with the arrival of 5,000 settlers in Natal (1820), and the Cape annexed territory south of the Orange River through successive wars with the Xhosa. Between Dutch and British colonies, a jumble of tribal Bantu and Griqua principalities created a rather unstable buffer zone. North of Natal, Zululand was by far the most powerful remaining tribal state.