China gained a strategic victory over the United States in disputes over the islands of Quemoy and Matsu in 1954 and 1958. Both times, China shelled the islands with artillery but backed down after the United States threatened nuclear retaliation. However, in 1960 the United States suffered a loss when Soviet leader Khruschev produced evidence at a conference that the U.S. had been spying with a U-2 plane, despite U.S. denials, damaging relations between the two countries.