Complex sentence: The negotiations for the treaty were taken under the mediation of Theodore Roosevelt (for which he won the 1906 Nobel Peace Prize), both sides were seeking a peace--the Russians had been repeatedly defeated, but the Japanese were in considerable financial difficulties.
Split into simpler sentences: The negotiations for the treaty were taken under the mediation of Theodore Roosevelt (for which he won the 1906 Nobel Peace Prize). Both sides were seeking a peace--the Russians had been repeatedly defeated, but the Japanese were in considerable financial difficulties.