Humans had just surfaced at the Galactic Core Peace Assembly as a nearly powerless backwater people without much hope for survival.  Many races saw the Earth as easy pickings, and most of these powers sought to take Earth for themselves.  When Farback made known its own intentions for conquer, the others (through knee-jerk survival instinct) immediatly backed away.

    The Farback Grand Fleet closed in on Earth 500 ships strong.  Just as advance scouts were being launched, and as Orbital Artillery was being deployed, a massive wormhole appeared before the advancing invaders, too close to stop and too large to avoid.  The entire fleet was drawn through the hole and into a region of space unseen by Galactic eyes.

    An ultimatum was broadcast through the ships' comm systems so that all on board could hear: Stand down your forces, apologize to the humans and ally yourselves with Earth.  If these three demands are not granted, your entire planet and race will be erased from existance, never known by any race, never victorious in battle.

    As an example of their power, the mysterious beings reduced the crew of every ship by half.  The remaining crewmales had no idea that the disappeared had ever existed, but for the life of them could not find a reason why they were to conquer an entire planet with 1/2 a crew.
 
    The display of power well taken by the Command crew, the Farback vowed to uphold the alliance and to treat these humans as their brothers.

Battered and bruised, the Farback fleet went back home while issuing a full apology to the people of earth, and sending a simultaneous Declaration of Peace.

No other race has invaded since.

Two, the Secret Masters might have been preparing for this for a long time, utilizing all of the occult/psionic/technological power at their fingertips to teach a possible invader a harsh lesson.  This most likely would have caused great stresses within the power structure of the Conspiracy, and it may still be rushing to fill the gaps.

Three, someone else with the interests of humanity in their own interests has orchestrated this incredible display of power.  This is most likely the Grays, but could be anyone.  The Secret Masters don't know who did it, are very grateful it that it has happened,  but are not asking dangerous questions.