Hindsight
by Jack Williamson
Years ago, engineer Bill Webster abandoned Earth for the employ of the piratical Astrarch
far beyond the orbit of Mars; now the Astrarch is aiming the final blow at a defeated
Earth, and Bill wonders whether the gun sights he invented can spot—and
change!—events in the past.
— Michael Main
The tracer fields are following all the world lines that intersected at the battle, back
across the months and years. The analyzers will isolate the smallest—hence most easily
altered—essential factor.
“Hindsight” by Jack Williamson, Astounding, May 1940.