Case: Zulu was a Royal Manticoran Navy alarm signal that was to be sent from a Naval vessel or installation upon good evidence that an invasion of the Star Kingdom of Manticore was imminent.
The term evolved from a large-scale combat scenario developed by First Lord of the Admiralty Thomas Cazenestro in the 1530s PD, originally called "Zulu Omega". (MA2)
In 1901 PD, Commander Honor Harrington sent a Case: Zulu warning through the wormhole just before shots were fired in the opening phase of the First Battle of Basilisk. (HH1)
In 1921 PD, Case: Zulu was declared by Admiral Grimm of the Manticore Traffic Agency at the central Junction of the Manticore Wormhole, and sent to the Admiralty, Home Fleet, and the other termini when the Havenite Second Fleet dropped out of hyperspace on their attack run against the Manticore System. The Battle of Manticore followed. (HH11)