With the immediate objective of capturing Seoul, the offensive commenced on 22 April on two broad fronts: the main thrust across the Imjin River in the western sector held by the US I Corps involving 337,000 troops driving towards Seoul, and the secondary effort involving 149,000 troops attacking further east across the Soyang River in the central and eastern sectors, falling primarily on US IX Corps, and to a lesser extent on US X Corps' sector.