Sentence:  Either commander took speedy advantage of it—Hopton to make a swift diversion into Sussex and capture Arundel Castle (which was but a <WSD>by-blow</WSD>, for in a few weeks he had lost it again). 
Meaning of the word marked with <WSD> tags:  A blow struck to the side or from the side, as in swordplay; a secondary or incidental strike of any sort.