Complex sentence: In the summer of 1919 the local Resistance movement threatened the regime of the White Cossacks and the Japanese and it was decided to launch a massive offensive on partisan positions in Eastern Transbaikal.
Split into simpler sentences: In the summer of 1919 the Resistance movement in Trasbaikal threatened to the regime of White Cossacks and the Japanese. It was decided to launch a massive offensive on partisan positions in Eastern Transbaikal.