FOR WANT OF A NAIL

"This proverb, having numerous variations over several centuries, reminds us that seemingly unimportant acts or omissions can have grave and unforeseen consequences." (Wikipedia)

In our day, this idea is often referred to as "the butterfly effect."

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 message was lost.
For want of a message, the battle was lost.
For want of a battle, the kingdom was lost.
And all for the want of a horseshoe nail.