In 2010 the Chinese declared war on America after its refusal to allow the excess population of China to immigrate to America. The Chinese's infamous Red Army led the invasion of the American coast, a focal landing point of which was San Andreas.
San Andreas became a war zone. Civilians were evacuated from battles and hostile areas by the USMC and SANG, but many were killed or captured and tortured by the Red Army.
As such the Venturas Underground Army was formed by grieving family members, who wanted to end the war, by any means necessary. The VUA soon emerged as the most powerful of the new rebel groups popping up throughout America, however the American forces and British forces saw them as terrorists and fought against them.