When people look back at how Ireland went from the highs of a Grand Slam and beating the All Blacks in 2018 to the soul searching and naval gazing that followed another World Cup let down last autumn, the name of England will loom large.
Their Grand Slam defence lasted only one game as they were blown away by Eddie Jones' side. The likes of Manu Tuilagi, Maro Itoje and the Vunipola brothers ran riot that day, the abrupt return to earth and getting bullied so badly on their home patch having a lasting physical effect on the squad.
Indeed, as Ireland stuttered through the rest of last year's Championship, then head coach Joe Schmidt aired his concerns that his side were "a little bit broken" by what had transpired in a game not as close as the 32-20 scoreline would have had you believe.
While the stakes were not as high, worse was still to come when the sides next met.
A record-breaking hammering in Twickenham marked the first game of Ireland's World Cup preparations last August, the visitors undercooked and left shell-shocked to the tune of a 57-15 embarrassment.
Thanks to Ireland's win over Wales in Dublin, Andy Farrell's men will be heading to Twickenham with a Triple Crown on the line when the sides meet again in London on February 23.
Ulster captain Iain Henderson played in both those harrowing losses last year, although in a more positive memory was also in the side that won a Grand Slam in Twickenham in 2018.
"Obviously the Triple Crown is something that would be great to win," said the soon-to-be 28-year-old. "But what a lot of the players will be thinking about is the last game that we had in Twickenham and how that definitely didn't go the way we wanted it to go.
"We'll definitely be looking back at that and looking to see how we can make improvements and what we can do. England will definitely be up for it, they'll be looking to come out all guns blazing. We've a week off to regenerate, a week to recuperate and to fix what we think needs fixed and then really ramp it up going into the game week.
"I think when any team puts 50 on you, there's a wee element of surprise. Coming off the back of the preparation, it was a first game for a lot of players, there were a lot of different factors.
"We'll look back at that, we'll see what went wrong, but ultimately we'll be wanting to produce a very different performance."
Despite opening their campaign with a win over Scotland last weekend, it would have been a similar attitude when they prepared for Saturday's win over Wales.
Ireland knew they had to set a different tone physically and were able to do so, the 24-14 win the product of dogged and determined work by a forward pack that had been bested even in victory seven days prior.
"Any game you go into, you want to be on the front foot but it was an emphasis for us that we wanted to win a few more collisions," Henderson said. "Scotland put it to us last week.
"There's lots of improvements to be made but we're happy with the performance and happy that the lads worked hard for each other out there.
"After last week we were happy to get the win but we knew it was a building block. A few things didn't go right for us but it's another step in the right direction, another building block. We can leap into that game in two weeks.
"It's all about setting down a marker physically. Against Scotland we saw that we didn't do that and it became a lot harder than maybe it should have been.
"Here (against Wales) we laid down a marker early. Ideally we would have preferred to get more out of that first scrum, more out of that first lineout maul, but like I said there's stuff to work on and get better at.
"Dominating (England) physically early on will be key but everyone wants to do that in every game."
Saturday's performance was certainly a step in the right direction, the game's biggest defensive moment coming in the 56th minute when, not long after Hadleigh Parkes had a try chalked off for a fumbled grounding, the Irish forwards forced a penalty to relieve what had been the visitors' longest spell of pressure.
What had seemed sure to be only a five-point lead with Wales in the ascendency was instead Ireland still 12 to the good, having dealt a key psychological blow to their opponents. The delight of the Irish pack was etched across their faces as they emerged from the pile.
"I think when you see guys winning collisions, guys winning turnovers, it means so much to everyone around you," said Henderson. "I think we really relish seeing that, guys working hard.
"It's not even necessarily who gets the turnover, it might be the guy who gets the turnover in the phase before, that's where we give each other pats on the back, that's what we enjoy.
"That was definitely a key moment. Any time a team looks like scoring and you can deny them, it has the potential to be a turning point. For us to keep them out there, it was massive for our confidence and I'm sure put a massive dent into them."
Not long after that World Cup warm-up in Twickenham, wins over Wales assuaged fears and left some thinking a corner had been turned ahead of the trip to Japan. What followed showed the reality was anything but.
So while Ireland are presently a better team than they were painted as last week, but not as good as the one they'll be said to be this week, it's worth remembering that they remain a work in progress.
Henderson, for one, believes their best is on the horizon.
"The quality in training and the quality we see guys producing for their provinces, we know it's there and form is one of those elusive things in sport that no one can really explain," he said.
"But when you get it right it feels great playing out there, it feels great amongst guys who are excited to do things. Hopefully as this tournament goes on we can bring more and more of it."
They'll likely need that and more heading back to Twickenham.