Manchuria is already part of the People’s Republic of China. The area seized by the Russian Empire in the 1800s is the region just east of Manchuria (south of the Amur River)—taking that enabled Russia to have a port on the Sea of Japan (part of the Pacific Ocean), Vladivostok. Maybe there was no place north of the Amur River suitable for a major port.
WWII began with Japan’s invasion of Manchuria, it just took ten years for it to spread globally.