Some battles are fought behind closed doors, or in the privacy of a training ground or a dressing room, taking time for the enmity to seep out and become public.
The rivalry between Roy Keane and Alf Inge Haaland was, from when the first blow was landed, fought in full view, reminiscent of gladiators in ancient Rome.
This is a family newspaper but it's hard to tell the story of Keane v Haaland without a lot of asterisks. "I f****** hit him hard. Take that you c***. He was an absolute p****."
There was huge collateral damage from the two main incidents between the pair, first in September 1997 and again in April 2001. Keane reckoned the second clash, or in particular two words in the description of it in his first autobiography, "cost me four hundred grand".
Their first duel in '97 ended Keane's season just weeks into the campaign, the second one effectively finished Haaland's career as he barely played again (just six times) after a tackle from Keane in a Manchester derby (though Keane points out that Haaland was fit to play four days later).
It ended Keane's relationship with his biographer, Eamon Dunphy, and caused a rift with one-time Ireland teammate David O'Leary, Haaland's manager at the time of their first clash ("F*** off Dave, you sent that bastard out to wind me up," Keane wrote).
And through it all, Keane remained defiant. Writing about it in the second volume of his autobiography, a full 13 years later, the Corkman was not for shirking.
"There are things I regret in my life and he's not one of them. He represents the parts of the game I don't like," Keane said of Haaland.
There was no real history between the pair before that infamous Leeds United-Manchester United game in September 1997, the two previous meetings between Keane and Haaland (when he was at Nottingham Forest) passing off without incident.
It was the ninth game of the Premier League season, but Keane's last. He'd suffered a cruciate injury in a petulant clash with Haaland but, as United had already used all three subs, Keane stayed on the field, adding to the physical damage already done.
But it was the aftermath of the Keane tackle on Haaland which sparked a bitter war that would still be raging 17 years later, Haaland using Twitter as late as 2014 to make a pretty daft (and unfunny) joke about Keane's resemblance to Saddam Hussein.
"Throughout the game I'd been having a private feud with Haaland, he was winding me up from the beginning of the game," Keane recalled in his 2002 autobiography. Keane said he could "live with the tackles" but not Haaland pulling his shirt or "digs off the ball".
With five minutes left in the game, United's first defeat of the season, Keane tried to tackle Haaland ("I was trying to trip him up rather than kick him") but tore his cruciate in the process.
"The pain was instant and agonising," he said. "Haaland stood over me shouting, 'get up, stop faking it'."
The words stuck in Keane's mind even as his rehab progressed: "I hadn't forgotten his contempt as I lay on the pitch at Elland Road".
The anger stayed under the surface when the pair clashed again, Keane scoring in a 3-2 win over Haaland's Leeds side in November 1998 and an event-free Manchester derby in November 2000 (Haaland was now a City player).
But in April 2001, the niggle went nuclear in another derby. "I'd waited almost 180 minutes for Alfie," Keane said in his book.
"He had the ball on the far touchline, Alfie was taking the piss. I'd waited long enough, I f****** hit him hard. The ball was there (I think). Take that, you c***. And don't ever stand over me again sneering about fake injuries."
A dismissal and suspension didn't end Keane's woe as that one paragraph in his book landed him in deeper trouble, the accusation that he deliberately tried to injure Haaland but also, Keane has said in his second book, that he was "profiting from my description of the tackle."
Keane would say that his words were "artistic licence" by his biographer but when Dunphy was asked by an FA panel if Keane's intent was to hurt Haaland, he was clear.
"Eamon's three words back were: 'without a doubt'. That was the case, my defence, out the window," Keane said later in The Second Half.
He also came to regret the careless, jokey use of "I think" about whether the ball was there to be won or not. "There were just two words that cost me," he rued.
The pair never met again and Haaland struggled to make a full recovery before retiring. Haaland stopped short of blaming Keane for ending his career but did say, in 2008: "I never played a full game again, did I? It seems like a great coincidence."
In a 2008 interview, Keane said: "Haaland has acknowledged that my tackle did not bring his career to a premature end."
But Haaland still bore some resentment. "I don't blame him for kicking me in other games or that particular game. What I was concerned and worried about is that he said, in his first book, that he wanted to take revenge. And I don't think that's part and parcel of football," he told the BBC in 2014.
Keane maintained his challenge on Haaland was not premeditated, that if he was a "madman out for revenge" why would he wait four years to get it?
But Keane still couldn't hide his dislike of the Norwegian. "He was an absolute p**** to play against. Niggling, sneaky," he said. "Did I try to injure him? Definitely not. But I did want to nail him and stand over him and go 'take that you c***. I don't regret that."
Keane's rivalry on the field with Patrick Vieira defined the Premier League for a decade and that too became personal, but they became close, and Keane also established a working relationship with one-time foes Niall Quinn and Mick McCarthy.
But reaching out to Haaland remains untouched territory.
Our writers to recall their favourite sporting rivalries