Its from the George Herbert poem Jacula Prudentum meaning "short and pointed remarks made by judicious men" (circa 1625) There is controversy over whether it was an old saying rather than a Herbert coining the phrase.
For want of a nail the shoe was lost,
for want of a shoe the horse was lost,
for want of a horse the knight was lost,
for want of a knight the battle was lost,
for want of a battle the kingdom was lost,
all for want of a nail.
Basically a saying that means you have to look after the small stuff to make the big stuff happen.
He also coined (which I really like)
"Living well is the best revenge."
So to my answer, which is a hazardous guess
You're clues were
Nails, horses & cannons
British
Doody and you are inclined to know? I thought maybe that was cryptic - with the real meaning your online name Charge. Doody's - I was lost on that one???
So I think it was the French cavalry charge at Waterloo. The French captured the English cannon but they lacked any headless nails to spike the guns. The English which had formed infantry squares inflicted heavy losses on the cavalry and recaptured the guns. Had the French been able to destroy the guns, the battle would have likely turned out differently. Napoleon launched a frontal infantry assault against the English line and the same cannon they had been unable to destroy.
"for want of a nail the kingdom was lost".
The thing I can't figure out was that this battle was a 150 years after the phrase was written by Herbert. My best guess though