Complex sentence: By the end of that month, Finland's Commander-in-Chief, Field Marshal Mannerheim, was pessimistic about the military situation and on 29 February the government decided to start peace negotiations.
Split into simpler sentences: By the end of that month, Finland's Commander-in-Chief, Field Marshal Mannerheim, was pessimistic about the military situation. Therefore, on 29 February the government decided to start peace negotiations.