Six at the beginning means

In advancing and in retreating,

The perseverance of a warrior furthers.

Inborn gentleness is often carried to the point of indecisiveness. One does not feel strong enough to advance resolutely. A thousand doubts crop up; one is, however, not minded to withdraw but drifts indecisively to and fro. In such a situation, a military decisiveness is the proper thing, so that one resolutely does what order demands. Resolute discipline is far better than irresolute license.