It is a phrase that reflects the carefree Icelandic spirit and is used so frequently by the island’s 335,000 inhabitants that it has been described as the national motto: thetta reddast, ‘It will be OK’. Whatever happens, and however grave the situation, it will somehow work out.

“Why are Icelanders so optimistic?” Heimir Hallgrímsson, the Iceland manager, said. “I think it’s in the genes. We are optimistic about everything. If it’s the Eurovision Song Contest, we always think we’re going to win it, even though we never do. It’s the same here at the World Cup. We think we’re going to win all the games and, even if we lose, we think we’re going to win the next one.”

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Iceland did lose the last one – 2-0 to Nigeria – and they now know they have to beat Croatia on Tuesday, hope that Nigeria fail to win against Argentina and also earn themselves a better goal difference than Nigeria or Argentina to advance from Group D. A Nigeria victory would send Iceland and Argentina home.

Hallgrímsson is convinced qualification is on and not because Croatia, who are already assured of their last-16 place, are set to make wholesale changes. Hallgrímsson’s opposite number, Zlatko Dalic, has six players on yellow cards, including Ivan Rakitic and Mario Mandzukic, and he is expected to rest them all – together with his other leading lights, Dejan Lovren and Luka Modric.

“It’s strange to say but it would be a disadvantage for us if they rested players,” Hallgrímsson said. “The Croatia players who played the first two games could be demotivated – thinking about getting injured or yellow cards. Now it’s players who want to punch themselves in to play the last 16. This is their chance, so we will have a group of really motivated players coming in.”

Iceland have built a hardy mentality over recent years, which was in glorious evidence at Euro 2016, when they reached the quarter-finals having knocked out England, and Hallgrímsson says they have nothing to fear against Croatia, whom they finished above in qualification.

They had faced Croatia in qualifying for the 2014 and 2006 World Cups – they lost a two-legged play-off to them in 2014 and two group matches in 2006 – but this time, they could finally claim to have found the upper hand. After a 2-0 defeat in Croatia, they beat them 1-0 at home in June of last year.

Hallgrímsson’s team were poor in the second half against Nigeria and the game got away from them but they had been decent enough in the first half and resilient over the 90 minutes of the 1-1 draw with Argentina in their opening tie.“Beating Croatia gives us belief and our players deserve a lot of praise for finishing above them in qualification,” Hallgrímsson said. “We are up against the wall and we need a result from the game between Nigeria and Argentina but it’s all about us now. We have to do our job.”

The captain, Aron Gunnarsson, added: “We’ve been in this position before. In qualifying we lost away to Finland and we had to pep ourselves up but we then won our final three ties.”

Gunnarsson talked up the unquenchable spirit within the Iceland squad and he noted how “with us, it’s always a lot of fun”. One moment from the pre-match press conference illustrated the point. A journalist posing a question is required to identify the title for which he or she works and one Icelandic correspondent announced that he represented ‘Pink and blue porn magazine’. Gunnarsson could not suppress his laughter.

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The smallest nation to feature at a World Cup will stick to what they know; the routines and tactics that have served them well. Hallgrímsson is likely to set up in a 4-5-1 formation and demand that his players do not given an inch. There is also the expectation that the neutrals at the Rostov arena will back Iceland, given that they have three Rostov players in their squad – Ragnar Sigurdsson, Sverrir Ingason and Björn Sigurdarson.

Could Iceland really end up qualifying ahead of Lionel Messi’s Argentina? How would the country react? “The Icelanders would think that they would become world champions because they are so optimistic,” Hallgrímsson said. “Only half of the nations qualify for the last 16 and for some of the big ones not to make it would be a shock. If we qualified, it would be the biggest success in the short history of Icelandic football.”