The topmost line is the failure the whole hexagram exists to warn against. Legge's translation describes being all astray on the subject of returning, and does not soften what follows: evil, calamities and errors together, armies put in motion under such views ending in a great defeat that reaches the ruler of the state, and ten years insufficient to repair the damage. This is among the heaviest judgments in the book, and it is attached not to an error but to the refusal to turn back from one.
Distance explains the severity. The single returning light sits at the very bottom of the figure and this line sits at the very top, as far from the turning point as the hexagram allows, with the entire body of the upper trigram Kun lying between them. Every other line found some way back. This one has moved past the last position from which a return was still available, and the text treats that as a different order of fault.
The reference to defeated hosts and a ruined state extends the consequence well beyond the individual. Calamities from within and errors from without arrive together, which is the classical picture of a failure both self caused and externally punished. Return closes on this note so that its central claim cannot be missed: the fault was never the straying, and it was always the refusal to come back.