On New Year's Day 1975, Communist troops launched an offensive which, in 117 days of the hardest fighting of the war, led to the collapse of the Khmer Republic. Simultaneous attacks around the perimeter of Phnom Penh pinned down Republican forces, while other CPK units overran fire bases controlling the vital lower Mekong resupply route. A US-funded airlift of ammunition and rice ended when Congress refused additional aid for Cambodia. The Lon Nol government in Phnom Penh surrendered on 17 April 1975, just five days after the US mission evacuated Cambodia.