Si vis pacem, para bellum
(if wish peace, prepare war)
Literally: "when you hope for peace prepare for war"
(or more generally - when you hope for peace be ready for war)
Peace though Strength
Igitur qui desiderat pacem, praeparet bellum
Literally: "Then, anyone who longs/wishes for peace should ready war plans" and it is taken from "Epitoma rei militari" (also known as "De Rei Militari","Of the art of war") by Publius Flavius Vegetius, a writer of the 4th century a.D.
Also some variations on the theme:
Si vis bellum para pacem
If you want war, prepare for peace.
Applied to Bonaparte as an example of planning for a war by making other nations think you are wanting peace.
Could also mean that if you prepare for peace you are inviting another party to start a war with you.
Si vis pacem para pacem
If you want peace, prepare for peace.
If nations really want peace it is difficult to see this while they build up their armaments (e.g. Andrew Carnegie 1907 National Arbitration and Peace Congress).
Si vis pacem fac bellum
If you want peace, make war.
The only way to liberate the world from military domination can in the extreme case be through war, an extension of the original: Si vis pacem, fac bellum (e.g. Richard Grelling 1918 on what is needed to make the world safe for democracy (comment on Woodrow Wilson speech before congress)).