After WWII, both the US and USSR began developing their early rockets based on the German V-2 work. USSR flew their R-1 copy of the V2 in 1948; the US flew Corporal in 1947.
It wasn't until 1957 that the first artificial satellite, Sputnik 1, was launched into orbit.
Obviously, tension between military, political, and scientific goals had a lot to do with the priorities of both nations' rocket programs, but is there any particular technical reason a Sputnik-type satellite couldn't have been launched sooner?
The Sputnik orbiter itself wasn't fancy: a tough shell around a big pack of batteries and a radio transmitter, trailing whip antennas. Contemporarily with the 84kg Sputnik, the US was building the functionally similar Vanguard satellite in a much smaller package massing 1.5kg.
Sputnik's launcher was an R-7, a fairly complex, 20-combustion-chamber, one-and-a-half-stage ICBM design (amazingly, the same basic design is still in use today!). The two US satellite programs under development when Sputnik launched, Vanguard and Explorer, were using 3- and 4-stage designs with simpler, single-chamber, liquid-fueled lower stages and solid uppers.
I assume that building a reliable, staged launcher of sufficient power was the major technical hurdle, but the US had flown the two-stage WAC Corporal to 50 miles altitude as early as 1946.
Was closed-loop guidance used/required for Sputnik, Vanguard, or Explorer? For my purposes, precise guidance isn't required; any orbit that could be sustained for at least a day or two would be acceptable for "Sputnik-style success".