The phoney war is almost over.

They are unbeaten, but not undoubted.

Victories over Argentina, Japan, Chile, Samoa and Fiji have kept England's campaign purring steadily forward. Not all the wins have been convincing, but on each occasion they have ended up on the right side of the scoreline.

The caveat is that they have also been on the right side of the draw.

On Saturday night, the velvet rope drops and they bash shoulders with the tournament's big rollers.

South Africa - the defending champions, who enacted their own Parisian revolution last weekend by deposing the hosts - have long set the sort of standards to which England coach Steve Borthwick's men aspire.

The Springboks are a team of gargantuan power and exhilarating pace, whose patterns run along well-worn grooves. They can overwhelm opponents with diesel grunt up the middle or out-gas them with pace out wide.

Former coach Eddie Jones' attempt to add dimensions to England ultimately folded flat. After the last Rugby World Cup and that painful final defeat by South Africa, he hoped to mould a team that could change tack and flip tactics mid-match.

Instead the transition was fumbled and it was the victors who succeeded in augmenting their repertoire.

Jacques Nienaber, Rassie Erasmus, Mzwandile Stick - a long-standing coaching brains trust - take the credit for that. They have had the time that Borthwick, appointed at the end of last year, hasn't. And they have spent it well.

Since Japan 2019, they have overseen a gradual evolution of their backline, bringing through playmakers Manie Libbok and Damian Willemse and elevating sevens specialist Kurt-Lee Arendse into the Test game.

In the forwards, their deployment of game-changing impact replacements in the front row has changed the way Test rugby is played. And, while others have talked about the concept of hybrid players, they turned it into reality.

Saturday's replacement hooker is Deon Fourie. The 37-year-old converted from front row to back row nearly a decade ago, but, at this late stage in his career, he has reverted back to allow the Boks to carry extra cover elsewhere in the squad.

The detail in the Boks' game runs to reams of such small print and footnotes. England, by Borthwick's admission, are working to a simpler plan, hoping their headline strengths are punchy enough to make up the difference.

Whether they can or not divides opinion.

One Rugby World Cup winner - Joel Stransky - said he would "fall over backwards" in shock if South Africa lose.

Another - Will Greenwood - would be less surprised by an England win.

"They have been building towards this," said the former England centre about the current side.

"They will not go quietly into the night. If South Africa win it, they will have to be fabulous."

The perception gap is filled with imponderables.

How much did that tussle with France physically and emotionally drain South Africa, who are unchanged for their meeting with England?

How much belief has built behind the scenes of England's siege mentality?

If the rain falls as forecast, might a saturated playing field be a more level one?

Come Saturday evening, we will know.

If England win, their claim to be serious contenders for the trophy will be suddenly and vividly realised. The doubters and detractors will be answered and the plaudits fully deserved. Borthwick's faith in his side and his style will be vindicated in style.

Lose though - especially by a wide margin - and those pretensions crumble to dust and ashes.

Either way, England's day of reckoning has arrived. South Africa will take them to previously unknown rugby altitudes, the brutal heights where titles are decided.

It's time to step up or stand down.