Onward, thru the fog

You’ve seen this view before on this site, but normally there’s a hillside covered with houses and trees and stuff. The other morning’s fog was thick as.

And… did you know the title before? I vaguely thought it was from Charge of the Light Brigade or Gunga Din or some such. And therefore it would have been “through” instead of “thru”. But the first several Google hits point me in a very different direction…