If my memory serves me correct, John Chancellor co-anchored NBC's live coverage of the Apollo 13 space mission (with Frank McGee), and it has been claimed that Chancellor's performance during that in-space crisis was what eventually got him an anchor chair.
That's plausible, Joseph, although it must be remember that Chancellor, McGee, and David Brinkley functioned in a rotating capacity, with only two of them anchoring (sometimes one) on a given night, seven days a week, for the first year after Huntley's retirement. The Apollo 13 coverage probably earned Chancellor a spot in the rotation (he had been HBR
's senior correspondent since his return to the Peacock Network in 1968), but I think it was mainly by default that he wound up as chief anchor on NBC Nightly News
in 1971. Brinkley was tired of the anchor desk grind, having done it for 15 years, and decided to step back and do pre-recorded commentaries about 2-3 times per week. Meanwhile, McGee had been doing the local news on New York's WNBC in '70, and a spot opened up on The Today Show
with the departure of Hugh Downs, so he went there. The sagging ratings against Cronkite on CBS above all else influenced NBC to go with Chancellor solo, which lasted until 1976, when Brinkley returned as co-anchor with him.