Jerry Sohn's "The Seventh Order" (short story, alien invasion, free): Invasion of the machines...

An almost invincible robot has arrived on earth from a far away world called Zanthar. A 7th order robot; by comparison, humans are equivalent to 5th order.

He's an advance scout, before more of his kind follow. Idea is to perpetuate themselves - force humanity to dedicate itself to manufacture more of them. But humanity is not ready to give up so easily...

Fact sheet.

First published: Galaxy, March 1952.
Download full text from Project Gutenberg, or as part of the magazine it originally appeared in.
Rating: B. 
Author's real name appears to be "Gerald Allan Sohl, Sr".