The Space Defense Force, or SDF, was originally founded in Sol System by its member worlds (Luna, Mars, and the Belt colonies) to protect their interests - which, at the time, would've included the extrasolar colonies on Annyrion and Oceania - from both the theoretical outside threat of alien attack and the very real threat of pirate attacks.

The SDF's charter, while providing a broad blanket of security across known space, does NOT permit it to interfere in the internal affairs of member planets. So, for instance, if a planetary government or megacorporation hires mercenaries or sends a ship to harass shipping from another planet, they will intervene as far as combatting the threat of that specific ship or ships.

On the other hand, megacorporate “hostile takeovers” conducted from within, or changes of government that appear to occur from within will be noted, but not dealt with directly as long as any resulting conflicts remain confined to that world's sphere of influence. It is up to the diplomatic corps of individual member worlds to deal with situations such as these.

The primary base for the SDF is the asteroid 951 Gaspra, from which warships such as the Paladin and Aurora patrol known space. While the SDF does maintain an impressive fleet of capital ships and escorts, they cannot be everywhere at once, and it may fall to member planets to provide the initial reaction force when dealing with piracy. If a threat is sustained, or is in danger of overwhelming a particular world's defensive fleet, SDF will respond to restore peace.

(Note: The SDF is a relatively inactive faction, player-wise, and primarily exists “in the background” in the Furscape universe. While the threat of a large, well-armed ship coming to rain hellfire down upon a pirate who is harassing your free trader sounds nice, the fact that such a ship may be days away from you keeps things interesting, doesn't it? - Hagalaz)