Which country launched the world's first man-made satellite, Sputnik, into Earth's orbit on October 4, 1957?

Sputnik, the world's first man-made satellite, was launched by the Soviet Union in 1957. This came as a surprise to the United States, who had announced in 1955 that they intended to launch a satellite during the International Geophysical Year (IGY). The period of July 1, 1957, to December 31, 1958 had been designated as the IGY in 1952, due to the predicted increase in solar activity during that time. While the United States solicited input from other countries in order to produce the technology required to launch a satellite, the Soviet Union kept their research secret. When they launched Sputnik--a much larger satellite than the one planned by the United States--and then a month later Sputnik II (larger still, and carrying a dog), the US and its allies were uneasy. A successful launch of that size could also mean a military advantage for the USSR. In January 1958 the US successfully launched Explorer I, and the space race was on.