The experience of persecution during the Civil War seemed to emphasize to Brethren their belief in separation from the world (Romans 7:1-2). The costume that Brethren came to wear during the frontier years was intended to free them from the styles and arrogant ways of the world. Like baptism, the costume meant that a person was seeking to live in the teachings of Jesus simply and directly. It began as a rejection of the elegant styles of the eighteenth century, and was later shaped by reaction to military styles. The mustache was discouraged at a time that it was fashionable in military circles. The first mention of dress by an Annual Conference was in 1804, which declared that "The new fashions which are in vogue in the world (grieve) God and the angels in heaven."
Before the Civil War Brethren generally encouraged one another to dress in a simple plain manner, but after the Civil War the Annual Conference began to indicate just how clothes were to be made and how hair was to be cut and combed. The question of how much pressure could be put upon church members to live according to the decisions of Conference became a very important issue. The efforts of Henry Kurtz, John Kline and others to create a common mind in the church failed.
Source: A Self-Instruction Guide through Brethren History, Donald E. Miller