horseshoe nail

The term “horseshoe nail” relates to a poem “For the want of a nail” from 1629 about how ultimately the fall of a kingdom was attributable to the consequences of a single missing nail in a horseshoe. The full text of the poem is as follows:

For want of a nail, the shoe was lost;
For want of a shoe, the horse was lost;
For want of a horse, the rider was lost;
For want of a rider, the battle was lost;
For want of a battle, the kingdom was lost,
And all for the want of a horseshoe nail.

Source: https://www.poetrybyheart.org.uk/poems/for-want-of-a-nail