Which country sent the first artificial satellite into orbit?

After the launches by the USSR of Sputnik 1 (October 4, 1957) and Sputnik 2 (November 3, 1957), the US finally succeeded with Explorer 1 on January 31, 1958.

Sputnik 1 had already fallen from orbit, and Sputnik 2 did so as well in April, 1958, following the March launch of Vanguard 1, the second US satellite and the oldest satellite still orbiting the Earth.