World Cups won: Nil
World ranking: 1
Coach: Joe Schmidt
Outlook: The Irish have never progressed beyond the quarter-final stage, but they will start their World Cup campaign as the world's best team for the first time in their history.
The team's optimism is guarded, though, having already lost three times this year, twice to England and to Wales, but their relatively weaker pool opponents, aside from Scotland, does allow Schmidt's players to reset for bigger tests later on.
Player to watch: Johnny Sexton
The reigning World Rugby Player of the Year enters his third World Cup in not the greatest of forms, having featured in only one of four of Ireland's warm-up matches due to injury.
But much will depend on the influential 34-year-old fly-half's experienced half-back partnership with Conor Murray if the Irish are to break their trophy duck.
World Cups won: 1987, 2011, 2015
World ranking: 2
Coach: Steve Hansen
Outlook: Although New Zealand underperformed in the Rugby Championship, their last two results before their World Cup defence have compelled - 36-0 over Australia and 92-7 against Tonga - and the All Blacks remain the bookmakers' favourites in Japan.
The squad have world-class depth in every position and maybe finishing third in the Rugby Championship was the wake-up call needed to focus minds on the quadrennial tournament.
Player to watch: Sevu Reece
The winger is a man who knows how to make the most of a second chance after he was charged with assaulting his girlfriend last year.
His call-up to the squad was contentious - he has since been discharged without conviction - but Hansen has backed the 22-year-old to become a ''better'' person, and his faith has been rewarded with two tries in three Tests.
World Cups won: 2003
World ranking: 3
Coach: Eddie Jones
Outlook: Until last year, England looked to be New Zealand's biggest rival heading into the World Cup, but the inconsistency of the last 18 months have asked some questions of Jones and his style of management.
The English have the ability and strength in depth to be world champions again, but are they strong enough mentally? Even the Australian coach is not sure.
Player to watch: Manu Tuilagi
A genuine game-changer, the 28-year-old can barge over the gain line and punch holes in the meanest of defences, with his ability to play at inside or outside centre.
England must pray Tuilagi can stay fit as a midfield axis of him, captain Owen Farrell and fly-half George Ford will give them a bit of everything outside the scrum.
World Cups won: 1995, 2007
World ranking: 4
Coach: Rassie Erasmus
Outlook: The Springboks have recovered from their two-year slump from 2016-2017, lifting last month's Rugby Championship for the first time in a decade. They have the elements to be competitive in Japan with a strong forward pack, while Erasmus, who has turned the team around, will also be key to their chances. Doubling up as South African rugby union's director of rugby gives him the latitude to make unconventional decisions.
Player to watch: Pieter-Steph du Toit
The flanker, 27, comes from strong rugby stock - his father Piet was also a Springbok - and he is a twotime South African Player of the Year. Du Toit started out as a lock, but was moved to the side of the scrum and has been a revelation, cementing his place in the South African backrow with a string of tireless performances.
World Cups won: Nil
World ranking: 5
Coach: Warren Gatland
Outlook: The Six Nations champions are primed and ready, with a game built around one of rugby's most relentless defences.
Wales look to strangle opposition attacks and score enough points to win through penalty kicks and strike runners like George North. Led by Gatland, widely regarded as one of the sharpest coaches in the business, they will be expected to at least reach the final eight.
Player to watch: Liam Williams
One of the top full-backs, he offers the perfect blend of defensive solidity and counter-attacking guile, exemplifying Wales' rope-a-dope style.
Williams, 28, can become the first player to clinch a domestic (club side Saracens landed the Gallagher Premiership and Heineken Champions Cup double), European, Six Nations and World Cup sweep in the same year.
Champions: 1991, 1999
World ranking: 6
Coach: Michael Cheika
Outlook: The Wallabies were buoyed by a 47-26 home win over the All Blacks last month, but were demoralised a week later after losing 36-0 away to leave the Bledisloe Cup with New Zealand for the 17th consecutive year.
Perhaps a bigger loss this year was full-back Israel Folau, who had his contract terminated by Rugby Australia over contentious posts on social media. And while Kurtley Beale has stepped in, he has shown signs of susceptibility in the wet. Australia's chances depend on how their scrum handle the set-pieces.
Player to watch: Jordan Petaia
The uncapped 19-year-old winger had his Test debut delayed because of continuing fitness problems, but he is set to become the Wallabies' youngest World Cup player.
Not a direct replacement for Folau, but his power, pace and threatening runs are still a genuine threat.
ASSOCIATED PRESS, REUTERS