responsible for reconnaissance, surveillance, the interdiction of the rivers and lakes to the enemy, and to avoid their use for the infiltration and supply of guerrillas in the interior of Portuguese territory. Additionally, the riverine forces were also tasked with the mobile fire support to the land forces, the movement of troops, the supply of the Portuguese garrisons and the support of the civilian population in the riverine areas. For these riverine forces, the Portuguese Navy conceived five types of vessels: the LFG (large river patrol boats of 200–300 t), the LFP (small river patrol boats of 18–40 t), the LDG (large landing craft of 480–550 t), the LDM (medium landing craft of 50 t) and the LDP (small landing craft of 12 t). The LFGs were armed with 40 mm guns and the LDPs with 20 mm guns, with several units of both types being also armed with rocket launchers. The LDG, LDM and LDP types were based, respectively, in the LCT, LCM and LCVP/LCA designs, but were modified in order to have a greater mission endurance and to be used for patrolling, fire support and as a mobile base for the Marines. This modifications included the protection of sensitive parts with armor, the installation of 40 mm (LDGs) or 20 mm (LDMs and LDPs) guns and the improvement of the crew accommodations, partially at the expense of the cargo deck. The river boat flotillas were complemented by assault units of Special Marines (fuzileiros especiais) and security units of Marines (fuzileiros). The Portuguese Marines operated based in the patrol boats and landing craft and also using their own rubber boats. Vietnam War On 18 December 1965, for the first time since the American Civil War, the United States Navy formalized its new, brown-water navy in Vietnam. Initially, the brown-water navy patrolled the inland waterways of the Mekong River, primarily with South Vietnamese river craft (RAG—River Assault Groups) boats, which were mostly inherited from the French Navy during the previous war and in turn, had been received from the U.S., as military aid, in the French fight against the Viet Minh, the Communist-led Vietnamese alliance. As the new fiberglass Patrol Boat, River using water jet propulsion, became available, it became the main interdiction vessel for patrolling the Vietnamese Mekong River country. For coastal duty the South Vietnamese Navy used larger seaworthy craft. These were replaced by newer U.S. Navy Swift