What was the first successful two-stage-to-orbit launch by the US? What was the first attempt if different?

Question

Wikipedia's Juno I (the launch vehicle for the first US satellite in orbit Explorer 1) begins:

The Juno I was a four-stage American space launch vehicle, used to launch lightweight payloads into low Earth orbit. The launch vehicle was used between January 1958 to December 1959.

and

Wikipedia's Vanguard_1; Mission says:

On 17 March 1958, the three-stage launch vehicle placed Vanguard into a 654 x 3969 km, 134.27 minute elliptical orbit inclined at 34.25°.

This is confirmed by the video 1957-1959 IBM 704 Computer - Vanguard Satellite Program "Science in Space" (Burroughs Datatron) NASA (alternate version of the video here) and screenshot below.

Question: What was the first successful two-stage-to-orbit launch by the US? What was the first attempt if different?

See also Was the R-7 the first two stage rocket of the Soviet Union?

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Screenshot of illustration of Vanguard 1 3rd stage within 2nd stage, from "1957-1959 IBM 704 Computer - Vanguard Satellite Program 'Science in Space' (Burroughs Datatron) NASA" link

Answer 1

On December 18, 1958 a modified Atlas ICBM flew to orbit.

https://en.wikipedia.org/wiki/SCORE_(satellite) 1958-006A

One may consider this either a single-stage-to-orbit vehicle that dropped engines, or a stage-and-a-half vehicle. Rounding up, two stages?

Expanding on the early Atlas configuration: These vehicles lifted off with three engines burning, but later in the ascent, they shed the two outer engines with associated plumbing and structure. The center engine continued to burn throughout the entire ascent.

Image from here (But it's from the old Flight International article on the Atlas)

And an "artist's impression" from here