So, what happened? By now we've sifted through a lot of post-mortems and, while this is a new week, it's worth taking a closer look back at how this Irish team was so badly taken apart. Those who fail to learn from history are condemned to repeat it, and there are major lessons in this defeat.
Before the specifics, a general issue: Japan were ready and willing for this game and Ireland weren't. I wouldn't call it complacency, but I don't think Ireland were thinking about Japan all summer whereas Japan were likely thinking about Ireland - this win could be a ticket to the quarter-final.
In 2014, we caused the biggest shock in the history of the women's World Cup when knocking out New Zealand, and a major reason was the psychological and physical preparation: we viewed it as our final from a long way off, whereas the Kiwis would have had one eye up the road.
Japan had an eight-day turnaround after the first game and they will have targeted Ireland with their six-day turnaround. It's not to excuse a bad performance, but part of the explanation is the style of rugby Ireland play - the one they played against Scotland - is hugely attritional.
Joe Schmidt speaking out about referee Angus Gardner before the game was also a bad idea. It can work in a team's favour, but it was a mistake because Japan were deep cleaning at the ruck and Gardner failed to call it. It's possible a referee can have an unconscious bias after being publicly called out on his performance during the Wales game in the Six Nations. It's not why we lost - Japan were better - but I don't understand Joe's rationale.
From a tactical standpoint, Japan figured out how to stop Ireland's one-out game by controlling possession and playing with tempo, going wide and moving the ball off turnovers and when they received kicks.
They stretched Ireland's defence and traditionally the edge is where we struggle to defend. Japan's defence was immense. They won the collisions when Ireland played their one-out rugby in Japan's 22 in the second half.
Usually Ireland are so good here, but Japan drove them back which meant on the next phase Ireland were taking the ball statically and could not gain any momentum with their usual style.
Ireland played some nice rugby in the first 20. There were brilliant breaks from Garry Ringrose, a slick sequence of offloads between Jacob Stockdale, Ringrose and Rob Kearney, while Jack Carty's willingness to pass to the outside channels was causing problems.
But after half-time there was a shift: we started to kick the ball away and Japan continually ran it back, no matter how deep. Did the half-time talk advise them to go back to how we usually play? In the first half it was attacking, offloading rugby, playing with tempo, but then we retreated within ourselves: low-risk, with a lot of kicking.
Japan's tactic of holding the ball and not kicking it back starved us of possession. Ireland didn't defend the fast-tempo rugby well while Japan wasted no time at rucks, which prevented Ireland setting up and getting into their aggressive rush defence. Japan also used the latch effectively which was effective at getting them over the gain line. They play very attractive rugby. They're highly conditioned and the way Japan play requires a high fitness level. Jamie Joseph backed that to tell from 60 minutes and they scored right on 59 minutes.
Japan kept getting the ball to the edge and Ireland's kicking to a very dangerous back-three was curious. Why do this? The players and coaching staff should have been able to notice it wasn't working yet we still did it. I think Ireland weren't used to teams running from deep and were expecting Japan to kick it back. When they didn't, we never adapted.
Our penalty count and indiscipline cost us, but Japan played so well that they forced us into desperate measures. Peter O'Mahony was guilty several times. In the last 15 minutes Ireland just looked desperate, a lack of energy and a lack of ideas. Where do we go from here?
First up, we have to recover as a team and the fact that there are only five days until Russia is a good thing. When you lose a big game like that you are keen to rectify the issues as quickly as possible. It's also time to rest several players. They badly need it.
The important thing is that Ireland see this game for what it is: a setback. But if they learn from it they can and will progress.
Rugby World Cup 2019
Donal Lenihan said on RTÉ before the game that down at pitchside the difference in size between the two teams was really conspicuous. Ireland were taller, heavier and stronger and, while this wasn't news in itself, it confirmed pre-match expectations that the visitors would overpower their smaller hosts.