“War is just when it is necessary; arms are permissible when there is no hope except in arms.”
This is a quotation of Titus Livius IX:1 iustum enim est bellum quibus necessarium, et pia arma ubi nulla in armis spes est) that Machiavelli uses in Ch. 24 of Discourses on Livy; Machiavelli similarly writes that "The justice of the cause is conspicuous; for that war is just which is necessary, and those arms are sacred from which we derive our only hope." (The Prince, Ch. 26)
Misattributed
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Niccolo Machiavelli 130
Italian politician, Writer and Author 1469–1527Related quotes

Cheers.
Speech in Hanley (4 January 1910), quoted in The Times (5 January 1910), p. 7
Leader of the Opposition

Speech to Conservative Rally at Cheltenham (3 July 1982) http://www.margaretthatcher.org/document/104989, regarding the Falkland Islands War.
First term as Prime Minister
Context: When we started out, there were the waverers and the fainthearts. The people who thought that Britain could no longer seize the initiative for herself. The people who thought we could no longer do the great things which we once did. Those who believed that our decline was irreversible—that we could never again be what we were. There were those who would not admit it—even perhaps some here today—people who would have strenuously denied the suggestion but—in their heart of hearts—they too had their secret fears that it was true: that Britain was no longer the nation that had built an Empire and ruled a quarter of the world. Well they were wrong. The lesson of the Falklands is that Britain has not changed and that this nation still has those sterling qualities which shine through our history. This generation can match their fathers and grandfathers in ability, in courage, and in resolution. We have not changed. When the demands of war and the dangers to our own people call us to arms—then we British are as we have always been: competent, courageous and resolute.

Source: An Economist's Protest: Columns in Political Economy (1966), p. 121

“When the shooting starts would you rather be armed or legal?”
Source: The Road

“war is a matter not so much of arms as of money”
Book I, 1.83-[2]
History of the Peloponnesian War, Book I

"Lessons of the Commune" http://www.marx.org/archive/lenin/works/1908/mar/23.htm, Collected Works, Vol. 13.
Collected Works

“When judges shall be obliged to go armed, it will be time for the courts to be closed.”
Said while travelling to California, having been advised to arm himself while there (1889); reported in J.K. Hoyt, The Cyclopedia of Practical Quotations (1896), p. 129.