In article .com,
Jim Conway wrote:

Joseph,
Thanks for the reply. The lamp post has been inactive, but presumably
in a working state (all existing wiring remained unchanged by me). I
only added a new lamp to the post to get it working again (and wired
Black to black, white/white, ground/ground - pretty straightforward).

The problem shows when I flip the switch, the lamp lights briefly, and
then goes off. Basement breaker is tripped at that point. There is no
GFI in the circuit...Breaker resets OK until switch flipped again. I'm
just wondering if the most likely scenario is an underground short
(perhaps from edging or aerating), or if my black/white could be
crossed at the panel, or maybe the new lamp itself.

Seems to me that the light was "inactive" because of this gremlin. Doubt
that your efforts caused the problem, but merely revealed it.