The Borg have changed their M.O. While their massive cube-ships can lay waste to entire Starfleet task forces, they cannot prevail against the combined, united quadrant fleets. Not to be thwarted from imposing total technological communism on the galaxy, they are employing a newly assimilated tactic.
Assimilation Drones come in various forms. Torpedoes, missiles, bots, subspace limpets, cloaked drones, cluster pods... all have the same end function. They invade any technological structure, be it a spaceship or a megacity. No different than a virus infection, they splice into power and computer systems to begin mutating them into Borgness.
This 'infectious' invasion is difficult to detect in comparison to Borg ships. Starfleet's "TeepNet" couldn't detect their approach. (TeepNet is Starfleet's division of telepathic sensitives who can easily sense the approach of the huge Borg technological gestalt of minds.) The purely robotic Borg weapons have no living mind to detect (until they begin assimilating).
The Federation has a huge border to watch. Storms, anomalies, nebulae, all manner of astronomical events can conceal travelers. (Ask any smuggler.) But the greatest threat posed by assimilation pods comes from terrorists. Despite the fact that 99% of civilizations would have nothing to do with the insanity of inviting Borg assimilation (even against enemies), there's always that 1% of nutballs, psychopaths, and vendetta addicts who will. They just don't care. As a consequence, Borg torpedoes can show up by surprise anywhere, even Earth. In one instance, leaders of one planet were willing to gamble that Borg drones would assimilate a rival planet in a nearby solar system, then be destroyed by the Federation before the assimilation could spread.
TeepNet is a Starfleet Division which was originally created to counter the Borg. A number of races in the quadrant can innately sense the approach of a large collective of minds. One of the inherent weaknesses of the Borg is, a technological gestalt of that many minds is difficult to conceal.
The Talosians emerged from their self-imposed isolation late in the 24th Century to offer exactly such a service to the Federation. From that point on, no living Borg incursions of any size could slip into Federation territories unnoticed. (However, it would fall to technopaths to defend against purely robotic incursions.)
The frustration for Starfleet was, and continues to be, Talosians have their own version of 'non-interference'. Still somewhat isolationist, they refuse to participate in Homespace Security operations. They'll warn against the big threats, but they don't make themselves available for general operations.
Starfleet embarked on emulating the Talosian defence with other races. And while those races fall far short of Talosian capabilties, this new TeepNet division was able to prove useful in a number of situations. Admiral Tersian Truhour made sure that TeepNet functioned with enough supervision that it didn't turn into another Section 31 or a Babylon-5 Psy Corps.