At some point, most likely after the collective was formed, the Borg began to incorporate machine parts into themselves. Organic systems where replaced with mechanisms in order to increase the capabilities of individual drones. This synthesis of machine and organic is thought to have been evolving for thousands of centuries; today Borg drones average more than 50% machine parts. There are many variations between Borg drones, with each presumably optimised for specific roles, but most drones share certain common features. Borg have a built-in replicator unit which can create or modify their mechanical systems as required and synthesize nutrients for their organic systems. There is usually a holographic viewing system replacing one eye, and a mechanical appendage replacing one arm. Drones carry a supply of nanoprobes which they inject into other lifeforms; these nanoprobes rapidly begin to construct machinery in the victim, beginning the assimilation process. The Borg rely on an onboard power cell for energy, and this must be recharged periodically. The two features shared by every known Borg are a shield system and a subspace transceiver which keeps the links between the drones intact; it is this network of links which comprises the collective itself.
In 2364 a Borg vessel is known to have passed
through both Federation and Romulan space, destroying outposts of both
powers. The Borg chose not to attack the Federation at this time, presumably
because they where on their way to some other more urgent location. In
2365 the Enterprise-D was flung into system J-25 by Q, where they encountered
a Borg vessel. The Borg sent scouts to probe the Enterprise - the first
of these was quickly killed, but subsequent drones proved to have a shield
device which was immune to phaser fire. The Borg ship subsequently attempted
to assimilate the Enterprise, but their cube was quickly disabled by the
Federation vessel. However, an away team sent to the Borg ship discovered
that the collective was rapidly regenertaing its ship. The Enterprise attempted
to depart the area before further conflic ensued, but the borg gave chase.
As with the drones, further attacks by the Enterprise proved to have no
effect on the Borg vessel. Later analysis showed that when the a Borg is
attacked the collective mind analyses the destruction of individual drones
and creates a defence, which all drones then add to their own systems
via their replicators. Their ships operate on similar principles. The
Borg vessel nearly destroyed the Enterprise, but Q intervened to save the
ship. However, the Borg have displayed a good degree of persistence and
since this time have launched two invasions of the Federation causing massive
loss of life on both occasions before being narrowly defeated.
Recently the Borg have encountered the life form they refer to as "Species 8472". Species 8472 proved both to be invulnerable to Borg nanoprobes and to have superior technology. In response to a Borg invasion of their space species 8472 nearly destroyed the Borg; with the help of modified nanoprobes from the USS Voyager the Borg where able to force Species 8472 to disengage and retreat from the conflict.