Complex sentence: Utrecht and Vryheid (the capital of the former New Republic) was then part of the ZAR until 31 May 1902 when the ZAR surrendered to Great Britain, after which both towns and their districts, were incorporated into Natal as spoils of war.
Split into simpler sentences: Utrecht and Vryheid (the capital of the former New Republic) were part of the ZAR until 31 May 1902, when the ZAR surrendered to Great Britain. After this, both towns (and their districts) were incorporated into Natal as spoils of war.