Northern Marchmen

Poul Anderson, "Among Thieves," see here.

Again we have compass points in space. See Galactic North And South etc.

"'...despite the fact that interstellar space is three-dimensional and enormous, Norstad guards the northern marches of Civilization.'" (p. 174)

Kolresh seizes a planet in the galactic west, raids one to the north, fights one to the south and allies with one to the east. However, if it by-passes Norstad, the latter will attack it in the rear.

Unduma lectures the Premier on the Terrestrial history of marchmen who guarded civilization against barbarians, then became barbarized and attacked the civilization:

Assyria and Mesopotamia;
Rome and Greece;
Welsh border lords and England;
the Transoxanian Tartars and Persia;
Prussia and western Europe.

Historical outcomes:

Assyria sacked Babylon;
Rome conquered Greece;
Percy rebelled and allied with Owen Glendower;
Tamerlane overthrew Bajazet;
Prussia attacked France;
Norstad-Ostarik will attack Earth.

In Anderson's Technic History, Dennitza nearly rebelled against Terra.