As the United Citizen Federation expanded its territory across the Galaxy, it came into contact with the Arachnid Species covering an area of it. The Federation initially considered the Arachnids to be a less advanced civilization. However, to avoid conflict, the region of space the Arachnids had colonized was Quarantined to prevent any humans settling within it. Despite this unofficial colonies were created, on Arachnid planets often ending in the Arachnid attacks of these poorly defended colonies leading to massacres such as at Port Joe Smith. In response to these intrusions into Arachnid colonies, the Arachnids launched an Meteor Attack on Buenos Aires. (Source: Starship Troopers wiki)
Battle vs. Xenomorphs (by Deathblade 100)
The battle starts with five Arachnid Warrior bugs, a Tanker bug and a Plasma Bug patrolling a rocky wasteland. The Arachnids enter a human colony only to find a strange black webbing covering the buildings. The Arachnid Warriors enter the main complex finding six humans cocooned on the walls.
Five Xenomorph warriors start to uncurl of the walls, angry that they have been disturbed. An Arachnid warrior moves into the hive and is attacked by the Xenomorphs. The Arachnid slashes its claws at one Xenomorph, killing it. The Arachnid's claws starts to melt due to the acid blood. A Praetorian rears back and spits two streams of acid at the Arachnid Warrior, blinding it. A Xenomorph thrusts with its tail and runs the Arachnid through, wounding it. The Arachnid contines to struggle as the Xenomorphs surround it and start to fire their inner jaws into the creature killing it.
The remaining Arachnids hear a skittering noise and see a Runner heading towards them. The Runner stops and spits acid at the Arachnids, driving the warriors towards the hive. A strange buzzing noise fills the air. A jet of blue plasma flies towards the hive complex killing one of the Xenomorph Warrior. The remaining Xenomorphs head out of the hive, swarming an unlucky Warrior bug. The Runner returns to the hive and spits more acid at the Arachnids. The Praetorian faces off against a Warrior bug, killing it with a head bite. The remaining Xenomorphs start to withdraw.
A Xenomorph Warrior charges towards a Tanker. The colossal Arachnid fires a jet of fire from its head, igniting the Xenomorph. The Xenomorph screechs in pain before exploding into acid, covering the Tanker in its blood. The three remaining Arachnids attack, killing the Runner. The Plasma Bug prepares to fire, just as the Praetorian slashes its claws through its abdomen and drives its tail through the Plasma Bug's head. An Arachnid Warrior bites down onto a Xenomorph's skull.
The Praetorian moves behind the wounded Arachnid and kills it with a headbite. The last Arachnid slashes its claws at the Praetorian, only for the Xenomorph rears back and spits acid into the Arachnid's face. A Xenomorph Warrior draws it's tail back and plunges it through the Arachnid's neck. The remaining Xenomorphs hear a noise and turn only to see a horde of Arachnids closing in on them.
The Xenomorphs had better X-factors than the Arachnids.
Battle vs. Tyranids and Zerg Swarm (by Deathblade 100 and Monkey Doctor 33)