protected by impassible swamp, marsh or lagoons and, where not, by of trenches with a garrison of 18,000 men. The river was shallow, uncharted and capable of trapping large vessels if the water level should fall. In that environment the greatest threat to shipping was "torpedoes" (nineteenth-century floating naval mines). Six vessels of the Brazilian ironclad squadron eventually succeeded in dashing past Humaitá in an incident known as the Passage of Humaitá, an event considered as nearly impossible. Although it could not operate far beyond its military forward base, nevertheless, Brazilian domination of the river meant that Paraguay could no longer resupply the fortress, and eventually it was starved out and captured by the land forces in the Siege of Humaitá. Even after Humaitá was captured − which took more than two years – the Paraguayans improvised further strongpoints along the river, further delaying the Allies (the Empire of Brazil, the Argentine Republic and the Republic of Uruguay). U.S. gunboat diplomacy in China Save for an occasional river patrol boat, the United States' river ironclad navy was all but abolished at the end of the American Civil War. Yet the concept of a river defense force lived on in countries and regions where rivers enabled the U.S. to project its military presence, allowing it to protect its foreign interests abroad. U.S. river boats (gunboats) of the Asiatic Fleet operated in portions of Chinese rivers, sometimes referred to as the "Asiatic Navy" or "China Navy", during the turbulent 1920s, patrolling for insurgents and river pirates. Two of the most notable China gunboats were , which was sunk in 1937 by Japanese military aircraft prior to World War II, and , which was captured by the Japanese in December 1941. The U.S. Navy of that era used the term for protecting U.S. foreign policy and its citizens abroad "gunboat diplomacy". The U.S. Navy, China gunboat, , was sunk by the Japanese in March 1942. Second Sino-Japanese War During the Second Sino-Japanese War, the Manchukuo Imperial Navy served principally to patrol the Sungari, Amur, and Ussuri rivers, support Army operations against Chinese resistance forces, and guard Manchukuo's riverine borders with the Soviet Union. In 1939 the Navy's forces came under the control of the Manchukuo Imperial Army as the River Defence Fleet. Indochina War During the First Indochina War, the French Navy created the Dinassaut (naval assault divisions), in 1947, to operate in