The Echo Wars
It was during the mid 26th century that the Echo Wars began. It was a brutal guerrilla style war on Mars and other colonies were Echoes hiding their identity attempted to solidify and then over through humanity. It was true war since humanity left earth which killed close to 50 percent of the populace of Mars. It is unclear how many Echoes still exist, but there have been several skirmishes with Cogs in outer colonies.