For Want of a Nail

"For Want of a Nail" is a proverb, having numerous variations over several centuries, reminding that seemingly unimportant acts or omissions can have grave and unforeseen consequences.
Analysis.
The proverb has come down in many variations over the centuries. It describes a situation in where there is a failure to predict or correct a minor issue; the minor issue escalates and compounds itself into a major issue. The rhyme's implied small difference in initial conditions is the lack of a spare horseshoe nail, relative to a condition of its availability. At a more literal level, it expresses the importance of military logistics in warfare.
Such chains of causality are perceived only in hindsight. No one ever lamented, upon seeing his unshod horse, that the kingdom would eventually fall because of it.
Related sayings are "A stitch, in time, saves nine" and "An ounce of prevention is worth a pound of cure". A somewhat similar idea is referred to in the metaphor known as the camel's nose.
Historical references.
 
The proverb is found in a number of forms, beginning as early as the 13th century:
Modern references.
Along with the long history of the proverb listed above, it has continued to be referenced in some form or another since the mid 20th century in modern culture. The examples below show how the proverb has had profound implications into a variety of issues and commentary in modern culture.