銀河辺境の攻防 = DEFENSE OF THE GALACTIC MARCH. Entomology: 銀河 (Ginga), Galactic or Galaxy (Milky Way). 辺境 (Henkyo), "Mark" or "Marches" (
a medieval border province, In contrast to a buffer zone, a march is typically dominated by a single given country and fortified for defence against the neighbouring country). の (No), a possessive. 攻防 (Koubou), Defense. But, the official English Subtitles for Episode 6, mentions the "Hinterlands"; so, perhaps that's what should be used (since it's canonical, as far as the English Subs go) as DEFENSE OF THE GALACTIC HINTERLANDS? Well, I'm still leaning towards DEFENSE OF THE GALACTIC MARCH...