For Want of a ...
A friend stopped by and commented, "If you give a mouse a cookie, he's going to ask for a glass of milk." Anyone know that children's story?

If a hungry little traveler shows up at your house, you might want to give him a cookie. If you give him a cookie, he's going to ask for a glass of milk. He'll want to look in a mirror to make sure he doesn't have a milk mustache, and then he'll ask for a pair of scissors to give himself a trim...

I was thinking about this while I went to Bible study and came upon the thought, "For want of a blank, the kingdom was lost. For want of a what?! For want of ....? Was it the kingdom or the war that was lost? I think it has something to do with a horse. For want of a horse the kingdom was lost? That still doesn't sound right." Though almost no one recognized giving a mouse a cookie... more gears started turning for want of a something. Is it Shakespeare?

A friend at Bible study got on Google and first found.

A horse! a horse! my kingdom for a horse!

which is from Shakespeare's Richard III. That is something but still I felt that was not it.
Then he found the elusive quote:

For want of a nail the kingdom was lost.

It is from a longer nursery rhyme–origin uncertain–likely English.

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.