Complex sentence: The Soviets were distracted by crucial events relating to their ongoing border disputes with Japan, needed time to mobilize the Red Army and they saw a diplomatic advantage in waiting until Poland had disintegrated before making their move.
Split into simpler sentences: The Soviets were distracted by crucial events relating to their ongoing border disputes with Japan. They needed time to mobilize the Red Army and they saw a diplomatic advantage in waiting until Poland had disintegrated before making their move.