Complex sentence: The Brazilian Empire's control over the Riograndenses had always been rather shaky--during 1836-1845 they had actually seceded, calling themselves the Riograndense Republic--so the Imperial government felt obliged to treat them with some caution: they might secede again.
Split into simpler sentences: The Brazilian Empire's control over the Riograndenses had always been rather shaky. Hence the Imperial government felt obliged to treat them with some caution: they might secede again.