TOKYO -- The frustration and hope in United States coach Gary Gold's voice was evident in equal parts throughout the 2019 Rugby World Cup.

His Eagles, who on Sunday signed off in Japan with a 31-19 loss to Tonga, resulting in a second straight 0-4 record at rugby's global showpiece, had performed admirably on each occasion, improving steadily across each match before they let a golden opportunity slip against the Pacific Islanders in Hanazono.

Leading 12-5 at halftime, the Eagles failed to mount any serious challenge in the second half as Tonga scored three tries to gallop away to a 12-point victory.

"We've spoken the whole World Cup about taking opportunities when you play good teams," Gold said. "Today there were some fantastic passages of play, particularly in the second half. None more so than one that went for 16 or 17 phases, we ended up a metre from the line and we didn't take the opportunity.

"Then you have a 14-point swing, that's what the game comes down to."

The Eagles' progression across their four fixtures was obvious to anyone with even a passing interest in the game of rugby. In their opening fixture against England, who were already battle hardened following a good opening hit-out against Tonga, the Eagles had the look of a team playing together for the first time: they weren't prepared to keep ball in hand, lacked confidence in their approach, had no answer to England's driving maul and kicking game, and only really threatened to score in the match's closing stages when the humid conditions under the Kobe Misaki Stadium roof really saw play open up.

But they took a significant step up against France, whom they pushed for 65 minutes to trail by just three points only for Les Bleus to run in three late tries, and by the time the Eagles faced Argentina in Kumagaya, they were far more settled in their structures and had a genuine belief in what they were doing.

There was no more of the aimless kicking that had been a feature of their clash with England, while hulking centre Paul Lasike was used to superb effect; the former Chicago Bears and Arizona Cardinals fullback hitting the line to provide front-foot ball for the following phase or producing an offload like the one that helped lay on the first of skipper Blaine Scully's two tries in Kumagaya.

Lasike first fractured the line having cut back on the angle, found an offload to fly-half AJ MacGinty, who produced a delicate kick in behind that sat up perfectly for the chasing Scully to touchdown. It was the play of a Tier 1 rugby nation: steady phase build-up finished with the ruthless execution that befits the eight teams remaining in the quarterfinals.

Argentina would eventually run out comfortable 47-17 winners, but the evidence of progression was clear and should have given them a launching pad to really attack the final game against Tonga.

"I thought the first 20 minutes were very, very good," Gold said of the Argentina clash. "Very similar to the French game, we put ourselves in a really good position, had an unbelievable opportunity to take advantage of the scoreboard and we didn't unfortunately score at that moment in time, but that's how rugby works; it's a game of momentum.

The USA were unable to pick up a single point in the tournament. Photo by Adam Pretty/Getty Images

"But we can see we've got a lot of work to do in terms of improving and that's why this group was a great group for us to play in; we're not feeling like that right now, but in reflection it will be a very profitable exercise. And this is what we wanted, we wanted to play against three teams [England, France and Argentina] who are top-10 teamsin the world."

The short four-day turnaround that preceded the Eagles' second-half fade against Tonga is a factor that shouldn't be overlooked, nor the fact the Islanders were motivated by skipper Siale Piutau's final Test match.

But when Gold talks about playing Tier 1 nations, the South African is right on the money. The only way this Eagles side will continue to improve is to get more regular fixtures against teams like England and France, or the likes of the New Zealand and Australia who toured the States before the previous World Cup, and to get a feel for the physicality, execution and match understanding that Japan have enjoyed over the past decade.

That responsibility is two-fold. Certainly World Rugby's overhaul of the global season from 2020 is a good start -- that will see matches between Tier 1 and Tier 2 nations increase by 39 percent through to 2032 -- but USA Rugby must make the nation an attractive place for the likes of Australia and New Zealand to head there for matches.

USA head coach Gary Gold admits that his side failed to make the most of their opportunities at this year's World Cup. Photo by Cameron Spencer/Getty Images

A Rugby Australia spokesperson told ESPN the Wallabies looked at a similar camp and match against the Eagles before this year's World Cup but the plan was "not commercially viable."

But there is hope for the Eagles, Gold and those at USA Rugby in what the hosts Japan have done over the past month.

The Brave Blossoms will this week contest their maiden Rugby World Cup quarterfinal just two tournaments after they could manage only a 23-all draw with Canada from their four games in 2011.

Now Japan have had the benefit of the nation's Top League, which has embraced a flood of players from around the world over the last few years, and strengthened in quality as a result, so too the amount of foreign coaches who have boosted the country's rugby IP.

But in Major League Rugby, which enters its third season early next year and has expanded to 12 teams with the addition of the New England Freejacks, Old Glory D.C [Washington D.C.] and Atlanta's Rugby ATL, the U.S. has a professional competition which should hopefully go from strength to strength, too.

Just last week, All Blacks great Ma'a Nonu signed on with San Diego Legion. Nonu is one of the biggest rugby names of the last decade and his move Stateside, even at 37 years of age, is a real shot in the arm for the American league.

Rugby in the U.S. is likely to improve when a number of quality players feature in the league next season. Photo by FILIPPO MONTEFORTE/AFP via Getty Images

And then there is the momentum that is building behind a USA bid for the 2027 Rugby World Cup. Gold was asked about it repeatedly throughout his team's time in Japan, so too the current tournament's effect on the Japanese people and what it could mean for rugby going forward.

"I think the quality of the World Cup in Japan has been absolutely outstanding," Gold said after his side's loss to Argentina. "I think seeing the Rugby World Cup in Japan has really struck a chord for me to believe that if USA were to be awarded the 2027 World Cup, I think you'll see very similar strides to the game in America as you've seen in Japan.

"I was here when Japan was awarded the World Cup, I was working here in Japan, that was followed by Japan obviously being very successful in 2015 and then in the last four years the game has just taken off. And the Top League has been absolutely outstanding here, the quality of the players has gotten better; a lot more of the Japanese players have become professional players.

"And that's exactly what will happen if they award the Rugby World Cup to America in 2027. The hype will grow, America will obviously host an amazing competition and I think it will be very good for the game."

Yes, there was plenty of frustration amid the Eagles' learnings in Japan. But there is just enough to inspire hope for the game Stateside, too, providing the right decisions continue to be made at home and abroad.