While Star Trek has had android characters for some time (most notably Data and his Soong-type brethren), and we do see that such androids take part in combat from time to time, I realized that I can't recall ever seeing any purpose-built artificial combat creatures - that is, creatures that are made specifically to fight in a similar way that ordinary biological lifeforms do, whether hand-to-hand, with melee weapons, or with ranged weapons like phasers or disruptors.
I thought of the following, but they don't really match:
- The Jem-Hadar. They are a purpose-bred fighting force, but they are specifically biological creatures, built by the Dominion using genetic engineering, not mechanical or robotic.
- The Borg. They are not all specifically bred specifically for combat, and their biological side is of critical importance in the story.
- Nanites. They are established as a civilization that finds itself fighting from time to time for their own survival, not a purpose-built army.
Did Star Trek ever portray an army of combat robots, androids, or droids in the manner of the Trade Federation droid army in Star Wars or similar? That is, I am generally talking about some civilization fielding a fighting force consisting of large numbers of mechanically assembled creatures purposely built for combat, or at least testing out small numbers of such creatures with an eye to building an army, not a civilization occasionally re-purposing an existing general purpose or otherwise non-combat android for combat detail. Also, android civilizations, societies, or bands that found themselves fighting for survival, fighting for their own political cause, etc., don't count unless they were originally built as fighting androids. Situations in which non-combat-specific androids, driven by plot elements, suddenly rise up and say "Hey, let's stop serving biological lifeforms, let's shoot them with phasers instead, long live metal!" are not what I am talking about.
If a civilization deployed purpose-built mechanical creatures in a quasi-military or paramilitary context such as law enforcement, that can count.
If an android civilization, fighting for a political cause, survival, etc., built new androids specifically designed to fight for them, that can count. For example, if a coalition of Soong-type androids built a new, combat-optimized Soong-type prototype and started testing it out with an eye to mass-produce them and use them to hijack starships, that would count.