sentences:  Along with U.S. Navy submarine chasers, chartered British naval trawlers, and fellow "Lapwing"-class minesweepers, "Flamingo" participated in the clearing of the North Sea Mine Barrage.Laid by the U.S. Navy after the United States entered World War I, the barrier had served as a formidable obstacle for German submarines based at North Sea ports.By 1919, however, the barrage merely hampered the resumption of peaceful commerce.