Complex sentence: Although it underestimated, till 1913, the political significance of the national question, as this significance manifested itself in the right of national self-determination, and its leadership kept a moderate position in regard with the nationalistic tendencies in Balkan social-democratic parties.
Split into simpler sentences: Although it underestimated, till 1913, the political significance of nationalism, as this significance manifested itself in the right of national self-determination. Its leadership kept a moderate position in regard with the nationalistic tendencies in Balkan social-democratic parties.