strike the kenote

[kjc0123]

What is the meaning of "strike the keynote"(specially, the literal meaning of keynote"?

In the first paragraph of his opening campaign speech, on June 17, Lincoln struck the keynote of American history for the seven years to follow:

A house divided against itself cannot stand. I believe this government cannot endure permanently half slave and half free. I do not expect the Union to be dissolved -- I do not expect the house to fall -- but I do expect it will cease to be divided.

[Ouisch]

A keynote is a major underlying theme or element.  Or, in the case of a speech, sort of like striking a nerve or chord - something that is meaningful to a large majority of people, something they can all identify with.  So when Lincoln said he was against a divided nation, he was echoing the thoughts of a large percentage of Americans at that time.