The Fish Speakers were an army created during the rule of God Emperor Leto Atreides II. The Fish Speaker army was entirely composed of women, with men having no roles in the Fish Speakers' lives other than that of husband. Men were totally shut out of the Fish Speaker combat forces, with the exception of the Duncan Idaho gholas. They were trained to be fanatical, disciplined, and extremely effective soldiers and police. The Fish Speakers were also used in Leto's breeding program, taking advantage of their speed and strength.
Leto II founded the Fish Speakers to enforce his rule over the known universe. He found the existing Fremen and Sardaukar forces unsuitable to his needs. Leto believed that male dominated military forces were essentially predatory and would turn against the civilian population in the absence of an external enemy. He also needed a reliable religious organization to enforce what came to be known as Leto's Peace to ensure the unfolding of the Golden Path.
With the exception of the nomadic Amazons who legend described as once ruling part of the Asian steppes of the Old Earth, the Fish Speakers were just about the only effective all-female army known throughout the history of the Dune universe. They were also, in terms of worlds ruled and historical longevity, the most effective military force ever known.
Post God-Emperor Rule
After Leto II's death, control of the Fish Speakers passed to the ghola of Duncan Idaho and Siona Atreides. Their influence had sigificantly waned in comparison to the Bene Tleilax, Ixians and the Bene Gesserit. A Fish Speaker council controlled some areas of space that used to fall under Leto's rule. By this time however, the Fish Speakers had also incorporated men into their ranks, and held little in common with the force maintained by Leto II, in terms of philosophy and practices.
Also by this time, many of the leaders of the Fish Speakers had been replaced with Tleilaxu Face Dancers, essentially making the Fish Speakers, and the realms under their domain, puppets of the Tleilaxu.