The "Vanguard Line" was a Galactic Republic nickname for a number of worlds in the Outer Rim Territories that the Supreme Modality fortified as a defense against the Republic's military incursions; the Modality called the line simply "the Barrier".
The Modality dropped a prefabricated garrison base on each Line world, consisting of the fortress and roughly ten thousand soldiers, as well as one or two Vanguardians. The Line was intended to force the Republic to either attrit itself against a number of tactically insignificant targets, or else focus on major targets and risk leaving Modality forces within striking distance of Republic worlds at its back. Modality troops also targeted non-Human populations on Line worlds. The Line stretched a vague arc from Garqi to Borgo Prime.
The Line held until mid-1,385 BBY, when a detachment from the Eighth Republic Expeditionary Fleet captured Phaeda. When the Modality did not retaliate, Darakhan launched Operation Liberator, which took Polus, Smarck, and Noonar, all as a guise for his conquests of Cademimu V and Agamar.