“As a soldier who has spent a quarter of his life in the study of the science of arms, let me tell you I went into the British Army believing that if you want peace you must prepare for war. I believe now that if you prepare thoroughly and efficiently for war, you get war.”

Quoted by Elad Benari in The Frantic Desire for Peace Only Brings War.
Source: http://www.israelnationalnews.com/News/News.aspx/140029#.VDs8U_ldUa4

Sometimes erroneously prepended to the opening line "The art of war is of vital importance to the State", but appears to be a variation of the Roman motto "Si vis pacem, para bellum". It's not clear who first misattributed this phrase to Sun Tzu. The earliest appearance of the phrase in Google Books is 1920, when it appeared in a pharmaceutical journal, but no attribution was given then.
Misattributed

http://www.nytimes.com/2006/03/27/international/europe/27memo.html?_r=1 Meeting with George W. Bush on January 31, 2003.
2000s

"Alex Jones EPIC Anti-Globalist Rant" https://www.youtube.com/watch?v=UHoc0FZG3OU, IncorporatedOps, February 13, 2017
2017

Source: Memory, Sorrow, and Thorn, The Dragonbone Chair (1988), Chapter 34, “Forgotten Swords” (p. 549).

Second term as Prime Minister
Source: Radio Interview for BBC Radio 3 (17 December 1985) http://www.margaretthatcher.org/document/105934

[Herrnson, Paul S., Women Running "as Women": Candidate Gender, Campaign Issues, and Voter-Targeting Strategies, The Journal of Politics, 2003, 65 no. 1]
2000s

On the morality of the firebombing campaign http://www.pbs.org/wgbh/amex/bomb/peopleevents/pandeAMEX61.html)