"Unarm" vs. "disarm"

Is there a semantic difference?

I thought unarm was more popular, but Google Ngram shows otherwise.

It seems to me the use of unarm as a transitive verb (to deprive someone of weapons, or to lay down one's own weapons) is at the very least archaic usage. Here's an instance from Shakespeare's Troilus and Cressida.

Unarm survives mainly as the past participle unarmed, meaning not carrying any weapons (particularly firearms).

The standard term for taking someone's weapons is to disarm, which carries much the same sense as in disarming a bomb, meaning to disable the detonator mechanism so it won't explode.

There is also the figurative use as in a disarming smile, where the sense is that one's potential hostility melts away in the face of a charming appearance or friendly behaviour.

The only dictionary that has "unarm" with the meaning of "To empty or strip of arms" is the OED and it gives it as Obsolete. Another one that has it is the FreeDictionary that defines it as a less common word for disarm and mainly used in the military.

In the OALD and in the NOAD, you can only find the adjective "unarmed" which means "not carrying weapons", used like this:

He was shooting unarmed civilians.

Disarm is the verb that appears in both (see the OALD again) which relatively to this question, means "to take a weapon or weapons away from somebody":

Most of the rebels were captured and disarmed.