forewarned is forearmed

Cf. L. praemonitus, praemunitus, forewarned, forearmed.

He that is warned afore is noght bygiled.
[c 1425 J. Arderne Treatises of Fistula (EETS) 22]

Once warnd, half armd folk say.
[c 1530 J. Redford Wit & Science l. 1093]

I giue thee this Ring of golde, wherin is written‥Praemonitus, Premunitus‥inferring this sense, that hee which is forewarned by friendlie counsoule of imminent daungers, is fore-armed against all future mishappe.
[1587 R. Greene Card of Fancy IV. 23]

Let all ships passing thereby be fore-armed because forewarned thereof.
[a 1661 T. Fuller Worthies (Devon) 272]

‘Forewarned is forearmed,’ he thought, as he complimented himself upon his success in baffling the attempt to ensnare him.
[1885 ‘le Jemlys’ Shadowed to Europe xxv.]

Forewarned is forearmed. Before the chemical disaster at Union Carbide's facility in Bhopal, India, the public was told an accident that has now claimed 20,000 lives could never happen.
[2002 Washington Times 30 June B4]

Related to: foresight and hindsight; prudence

Bibliography of major proverb collections and works cited from modern editions is available here.

Knowledge in advance enables one to be prepared, as in Let me know when he's in town so I can take the phone off the hook--forewarned is forearmed. This expression originated as a Latin proverb, Praemonitus, praemunitus, which was translated into English by the early 1500s. It soon was put to broader use than its original military applications.