United States and than the Soviet Union announced intentions to place a scientific satellite into orbit as part of the 1957-1958 International Geophysical Year, a World wide effort to study the Earth. Vanguard - a new vehicle descended from research rockets - was selected for technical reasons and also to emphasize the peaceful use of space.
The design specification for the Vanguard launching vehicle completed on 29 February 1956. Vanguard was a three-stage launch vehicle developed for the Navy in 1956-58.It had liquid-propellant engines in the first two stages and a solid-propellant third stage.It´s technical ancestors were the Navy´s Viking and Aerobee sounding rockets.The Vanguard test flight summary up to the successful launch of Vanguard 1 was as follows:

Flight summary up to Vanguard 1
Vehicle Objectives Results
8 Dec 1956
A liquid-propellant single-stage redesigned rocket. Fired as the first Vanguard test vehicle.
To evaluate the performance of the telemetry system,launch complex,flight attitude control system. and to become familiar with the tracking system and range safety. All objectives met excepted evaluations.Vehicle reached an altitude of 126.5 miles (205 km) and range of 97.6 mi (160 km)
1 May 1957
Two-stage test vehicle
To test the Vanguard 3d-stage and further evaluation of equipment and ground handling procedures All test objectives met.
23 Oct 1957
Prototype of a live 1st stage simulated 2nd and 3d stage
To test launch system and flight performance of the 1st stage. All test objectives met.
6 DEC 1957
First complete Vanguard test vehicle with three live stages.
To test and evaluate all stages of the vehicle, an "all-up systems engineerig test". Secondary:to launch a minimal satellite Vehicle exploded one sec after lift-off
5 Feb 1958
Identical to TV-3
Same as TV-3 After 57 sec the control system malfunctioned and the vehicle broke up.
17 March 1958
Identical to TV-3BU
Identical to TV-3BU Placed Vanguard 1 into orbit. Initial orbit apogee 2465 miles (3866 km), perigee 406 miles (656 km), period of 134 minutes.

It is worth noting that the TV-3 was a test-vehicle and if the 6 Dec -57 flight met all test objectives America´s first satellite was "a bonus". In the emotionally overwrought atmosphere of the early post-Sputnik era, however, it was perhaps too much to expect the public to distinguish between a promised launch and a promised orbit.So the December 6 launch-pad explosion drew all attention to the Soviet lead in space and started the "space race" that led to the Apollo program.