Legge's translation of the third line observes that there is no level place without its slope, and no departure without a return, and concludes that one who remains firm and correct amid the small hardships of a time already at peace commits no error, and need not grieve, since sincerity here still brings its own sustenance. The line states a natural rhythm rather than a specific danger, the plainest possible reminder that no settled condition, however genuine, holds forever without some answering change.
Nine in the third place sits at the top of Qian, the lower trigram, the last position before the hexagram crosses into Kun, the receptive earth above, and that boundary placement is exactly what gives the line its warning. Peace has been established through the first two lines' connected advance and broad, central conduct, but the third line stands where the ascending yang force must finally meet the yielding trigram it has been rising to join, and no line before it has needed to consider what comes after arrival.
Legge's commentary offers reassurance rather than alarm, holding firm through the ordinary difficulty a settled peace still contains, without despair, is itself sufficient to avoid fault. The line does not predict the peace will fail, only that its maintenance, like any level ground, will include some slope.