Now many people misattribute that quote to Sun Tsu, author of the Art of War.
It was certainly used by the young mafia leader Michael Corleone in 1974’s The Godfather Part II.
“My father taught me many things here — he taught me in this room. He taught me — keep your friends close but your enemies closer,” the Don told underboss Frank Pentangeli.
The elevation of Simon Coveney by Leo Varadkar to the position of Tánaiste following the resignation of Frances Fitzgerald has opened up a rear-side flank that Varadkar now has to constantly watch.
This weekend, Coveney stands emboldened, confident and resurgent while Varadkar, who bested his Cork rival for the leadership, stands weakened, diminished and with his judgement in serious question.
It was not supposed to be this way.
Varadkar was the new great white hope for the Fine Gael party after 15 years of Enda Kenny’s leadership.
His major selling points were his smarts and his ability to shoot from the hip.
Without question, Varadkar’s standout moment was when he broke ranks with Kenny, then justice minister Alan Shatter and the entire political establishment and described the actions of Garda whistleblowers Maurice McCabe and John Wilson as “distinguished” as opposed to “disgusting”.
As minister for transport, he had been away in the United States over the St Patrick’s Day celebrations in 2014 and came back to give an address on road safety at Dublin Castle.
McCabe’s allegations about an abuse of the penalty point system was the origin of his complaints.
He said: “Speaking on my own behalf and on behalf of the thousands of families who have had to endure the pain and loss that flows from the death of a loved one on the road, I want to thank Sgt McCabe and Mr Wilson.
"They may not have got everything right but they did shine a light into a dark place and forced those who would rather turn a blind eye to face up to the truth.
“There have been many words used to describe their actions. But if I was to use one word, the word I would use is ‘distinguished’.”
In that move, he wrong-footed his Taoiseach and senior party colleagues and caused a collective rethink to the attitude to McCabe and was the primary catalyst to the officer’s increased standing in the public consciousness.
It was also a major reason why so many in the Fine Gael parliamentary party decided there and then, that he was their guy to become leader.
Such clarity of thought has been so rarely in evidence since he has become Taoiseach.
His initial reshuffle was botched and underwhelming given he only demoted one person in Mary Mitchell O’Connor and missed a golden opportunity to put his stamp on the Fine Gael party.
The clearest sign of him pulling his punches was the decision not to promote John Deasy to the ministerial ranks, a move in his heart he knew had merit and also one he was clearlyconsidering given how widespread the speculation was about a Deasy elevation.
However, another sign of it was his decision to leave Fitzgerald as Tánaiste when most in Fine Gael considered her, like Michael Noonan, at the end of the road.
Given Simon Coveney had beaten Varadkar by a margin of two to one in the vote of ordinary Fine Gael members, his victory was not complete.
He has been plagued since by the valid accusation that he is a leader who is not endorsed by his own grassroots membership.
As embarrassed as Coveney was by the poor campaign he ran, the strong vote from councillors and ordinary members gave him a viable method to undermine Varadkar, if he so wanted.
So far, Coveney has not engaged in the sort of divisiveness that has previously engulfed major political parties after leadership contests.
Speaking to me hours after his appointment as Tánaiste, he said he has deliberately not done so.
“Leo and I are competitive people and neither of us like losing, certainly I don’t. But I was determined that once the leadership contest was over that I wouldn’t allow happen that which has happened in the past,” he said.
“That is that the divisions and bitterness of campaigns have dragged on which has damaged the party. I made a clear commitment to myself that I wouldn’t allow that to happen, to let that competitive tension fester. I simply moved on. Leo is the Taoiseach. He needs my support and he has had it ever since he has got the job,” he said.
“The evidence is clear, I have not done anything or said anything to undermine him. So for me, while I wanted a different outcome, I respected the outcome and moved on.”
I asked Coveney about whether he was surprised, as many were, that he was not appointed Tánaiste when Mr Varadkar first became Taoiseach in June.
“I am not going into conversations I had with Leo at that point. I live in the present. I regret on one level the circumstances around the vacancy but whenever you are asked to be Tánaiste it is a huge honour.”
Reading between the lines of what he said, two things are immediately clear.
Firstly, Coveney clearly expected to have been made Tánaiste in June and he was not happy when it did not happen. Secondly, his talk of “competitive tensions” is a clear signal that the dynamic between himself and Varadkar is not a cosy one and that the relationship is liable to be rocky.
I highlight this and compare it to Varadkar’s very close relationship with Finance Minister Paschal Donohoe which had become, prior to the Fitzgerald episode, the crucial axis upon which the Government had operated.
Fitzgerald was never a threat to Varadkar’s leadership and his retention of her in his first Cabinet had more to do with the past than the present or future.
The same can certainly not be said about Coveney, who definitely is a threat to Varadkar’s tanding.
His sure-footed performance at leaders’ questions on Thursday, minutes after his appointment as Tánaiste, gave a glimpse of what Coveney is capable of.
The loss of Fitzgerald means Varadkar has lost an ally but the decision to promote Josepha Madigan, a Leo loyalist, was a signal he was wary too of finding himself isolated at Cabinet.
“Frances’s replacement was always likely to be a woman and from Dublin. Given they backed Simon, that ruled out Kate O’Connell and Maria Bailey, so Josepha was the pick,” said one minister.
The Fitzgerald affair was not a fatal blow to Varadkar’s standing but he has allowed questions to be asked about his judgement.
He was on the verge of becoming the shortest-lived Taoiseach in history and if an election does happen in the spring, then he could still be.
Rejected by his grassroots and now forced to appoint his main rival to be his deputy, it has to be asked: Varadkar may lead Fine Gael but does he control it?