This is in order of importance to the teams' success, how the players in these positions can influence games during the knockout stages of the tournament.
This back three is undoubtedly the worst because of its inconsistency. I have no idea why they kept picking Yoann Huget. But if they play Damian Penaud, who is a massive star of the future, with Alivereti Raka and Maxime Médard at the back, they could cause some problems.
This is a difficult one. Marika Koroibete is class. Give him space and he will turn a game. Will they play Kurtley Beale or Dane Haylett-Petty at full-back? Does Reece Hodge come back in? Any back three that is not cemented should finish further down these rankings and the Wallabies are a prime example of that. As an Englishman, their backline does not fill me with fear.
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Similar to the other northern hemisphere teams, Ireland use their wings to chase box-kicks. Their skill levels need to be high in that area, and receiving high balls. Jordan Larmour is a fantastic athlete. I would stick with him, as well as the size of Jacob Stockdale and the defensive excellence of Keith Earls.
5. South Africa
When the Springboks have troubled New Zealand, their back three – and particularly Willie le Roux – have played well. Everyone knows about Cheslin Kolbe now. With Le Roux looking at blindsides and playing like another fly-half, doing unexpected things in tandem with Kolbe, South Africa are dangerous. South Africa's back three is not as important as their beefy pack, but they do make big impacts on games.
England's back three will not win them the World Cup, but they could lose it because of their aerial skills. Picking Anthony Watson on the right wing with Jonny May and Elliot Daly gives them an advantage there. It is a workmanlike back three. All three are exceptionally quick. Maybe they cannot change a game by themselves, but they can get on the end of things.
Josh Adams is the top try-scorer in the tournament, which shows how the team like to attack from right to left. George North's physicality is important. When he plays well and touches the ball a lot, Wales tend to as well. Liam Williams is brilliant in the air.
2. New Zealand
Any New Zealand back three loves playing rugby with broken-field ball. George Bridge and Sevu Reece are two more players who find space and take opportunities when they are presented. Give the All Blacks half a chance and that's all they need. I would like to see teams pressurise them more aerially. Kick in front of them and they'll destroy you. Let's see what happens if they kick on their heads. No one has bashed up Beauden Barett, yet...
If Japan are going to win the World Cup, it will be because of their back three. It is the most important component of their team. They are amazing to watch and play with such confidence. The offload from Will Tupou for Keita Inagaki's try at the weekend was brilliant. Kotaro Matsushima and Kenki Fukuoka have real pace and they do not mind playing in tight areas when Japan attack the blindside.