Complex sentence: In late autumn 1939, Ryti was offered post of prime minister but he tried to turn down the offer, but when the Winter War broke out on 30 November, Ryti agreed and he took his post on 1 December.
Split into simpler sentences: In late autumn 1939, Ryti was offered post of prime minister but he tried to turn down the offer. At the moment when the Winter War broke out on 30 November, Ryti agreed and he took his post on 1 December.