Complex sentence: From 1640 to 1729 they gradually worked out tenuous but stable diplomatic and commercial relations (for example the Treaties of Nerchinsk and Kyakhta), but their relations became a serious problem after 1858 when Russia annexed the Amur River basin and Vladivostok.
Split into simpler sentences: From 1640 to 1729 they gradually worked out tenuous but stable diplomatic and commercial relations (for example the Treaties of Nerchinsk and Kyakhta). Their relations became a serious problem after 1858 when Russia annexed the Amur River basin and Vladivostok.