Complex sentence: On 15 January, Carl Gustaf Emil Mannerheim, a former, competent, general of the Imperial Russian Army, was appointed supreme commander of the White Guards, and on 25 January the Senate renamed the White Guards the Finnish White Army.
Split into simpler sentences: On 15 January, Carl Gustaf Emil Mannerheim, a competent former general of the Imperial Russian Army, was appointed supreme commander of the White Guards. On 25 January the Senate renamed the White Guards the Finnish White Army.