UK History: Chapter 4, Exercise 1 — Possible Solution
======================================================================

THE BATTLES
25 Sep 1066, Stamford Bridge: Harold Godwinson defeated Harald Hardrada in
the north.
14 Oct 1066, Hastings: William's cavalry and archers overcame Harold's
infantry shield wall.

HOW THE FIRST MAY HAVE AFFECTED THE SECOND
The source says the victory left Harold's army weakened and far from the
threatened southern invasion.

CAUTION
The source does not say how much the weakening mattered, and I have not
checked the distances or timing.