MacBride's attempts are unsuccessful because the Empire assumes the Gogs cannot cross the sea barrier that separates the continents and, in any case, the Navy would stop them. MacBride's explanations that the sea barrier that separates the continents freezes over once every fifty years and that the freeze is due to happen again, giving the Gogs ready access to Avalon and making the Navy powerless to stop them, are ignored. Thus, the 19th-century-equivalent culture must contend anachronistically with an invasion of a barbarian horde similar to that of Attila the Hun or Genghis Khan (but armed with formidable, advanced weaponry). The nomads being named "Gogs" obviously links their invasion with the apocalyptic visions of Gog and Magog, derived from Jewish Eschatology and taken up by other religions as well.