The Second Klingon Empire was a period within Klingon history which ran between 2069 to 2369. (Decipher RPG module: Worlds)

The start of the Second Empire began after the death of the last Emperor of the Klingon Empire in 2069 which led to a brief civil war. This act resulted in the dimishment of the Emperor's power and the rise of the prominence of the Klingon High Council. In addition to this, the first Romulan-Klingon War ends at this time. (ST reference: All Our Yesterdays: The Time Travel Sourcebook)

Chancellor Mow'ga was one of the rulers of the Second Klingon Empire and ordered an armada of Klingon Empire to invade the territory of the Breen Confederacy. The fleet was never heard from again. (DS9 episode: "'Til Death Do Us Part")

Another noted period in the Second Empire was the peace treaty of 2290 forged by Chancellor Gorkon and President Eteon tar-Chereos that ended centuries of hostilities between the two governments. (Decipher RPG module: Aliens)

The unpublished Klingon Sourcebook Blood and Honor stated that the Second Empire began after the Hur'q invasion which suggests that the current Klingon government is the Second Empire however this should not be considered part of the source material.
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