Complex sentence: In January 1937 they began the "anti-Japanese expedition", sending groups of guerrilla fighters into Japanese-controlled territory to undertake sporadic attacks, while in May 1937, a Communist Conference was held in Yan'an to discuss the situation.
Split into simpler sentences: In January 1937 they began the "anti-Japanese expedition", that sent groups of guerrilla fighters into Japanese-controlled territory to undertake sporadic attacks. In May 1937, a Communist Conference was held in Yan'an to discuss the situation.