Complex sentence: Bulgaria, which had not participated in the invasion, occupied most of Thrace on the same day that Tsolakoglou surrendered taking the territory between the Struma river and a line through Alexandroupoli and Svilengrad west of the Evros River.
Split into simpler sentences: Bulgaria, which had not participated in the invasion, occupied most of Thrace on the same day that Tsolakoglou surrendered. The Bulgarians occupied territory between the Struma river and a line through Alexandroupoli and Svilengrad west of the Evros River.