Complex sentence: Daniel came of age in the 1270s and became involved in the power struggles of the principality with lasting success, siding with his brother Dmitry in his bid for the rule of Novgorod.
Split into simpler sentences: Daniel came of age in the 1270s and became involved in the power struggles of the principality with lasting success. Siding with his brother Dmitry in his bid for the rule of Novgorod.