On 31 May 1910, eight years after the Treaty of Vereeniging had signalled the end of the South African War, the Union of South Africa was formed as a self-governing dominion of the British empire. The new union, an amalgamation of two British colonies and two Boer republics, was presided over by former Boer General Louis Botha, its new Prime Minister. Botha's position was seen as a gesture of British conciliation to Afrikaner interests; in a similar spirit, the new cabinet included men of both nationalities. Sweeping political changes followed the country's unification. African leaders had held a convention at Bloemfontein in 1909 to discuss the implications of the new dispensation for their people. Over the next few years Africans would find their political rights and their access to land slashed, despite their appeals and petitions, while white workers intensified their demands for job protection and racial privileges.

Sidney was now moving in Labour Party circles. His friends Wilfred Wybergh and Frederic Creswell had joined the Party late in the day, but already Wybergh was a leading figure. In anticipation of national unity, plans for a unified Labour Party had been laid in 1908 and 1909; the South African Labour Party was launched as the first national-level political party on 10 January 1910. Affiliated to the Second International, it called for ‘socialisation of the means of production, distribution and exchange, to be controlled by a Democratic State in the interests of the whole community’ and ‘extension of the field of employment for white persons in South Africa’ – community meant white community.