Complex sentence: Lin Biao and other column commanders remained at their temporary headquarters 160 km away from Luding Bridge, and the task of taking the bridge was given to Lin Biao's vanguard unit, the 4th regiment, as it was 40 km closer to Luding.
Split into simpler sentences: Lin Biao and other column commanders had remained at their temporary headquarters 160 km away from Luding Bridge. The task of taking the bridge was given to Lin Biao's vanguard unit, the 4th regiment, as it was 40 km closer to Luding.