Iongi showed the touch of class which has seen him make the Brumbies Academy with two tries in each half as Tonga turned a 13-10 half-time lead into a comfortable canter on their Churchill Cup debut at Esher.
These two last met in the 2007 World Cup when Tonga won 25-15 - but this ranking Test match was far from close.
Tonga got off to a dream start when Iongi sped away from the grasping Andrew Suniula and raced away to the line.
Kurt Morath added the extras to an early penalty to give Tonga a 10-0 lead until Nesi Malifa pulled back three points.
But Morath restored the 10-point advantage after 21 minutes as the US Eagles' indiscipline again cost them.
Tonga lost prop Tonga Lea'aetoa to the sin-bin for a high tackle and the USA capitalised when Louis Stanfill rampaged downfield, Shawn Pittman took the ball on and when the ball was moved left, the experienced Paul Emerick dived over in the corner for Usasz to convert.
Scott Lavalla, consolation tryscorer in the 87-8 defeat by England five days previously, almost got on the scoresheet again but scrum-half Soane Havea got his hand in the way and ball was knocked forward from the second row's grasp to leave Tonga ahead by three points at half-time.
Usasz pulled an early second-half penalty wide across the face of the posts and things got worse for them when Eteuni Siua got away from Cornish Pirates-bound Roland Suniula and put Longi away for his second try.
Morath added the extras and then kicked his third penalty for a 23-10 lead when Pittman ended up in the sin-bin for a high tackle on Ilaisa Ma'asi.
The Eagles rallied, prompted by a probing kick from Usasz, but the attack came to nothing and it was Tonga who tightened their grip when Iongi wrapped up his hat-trick. Pepa Koloamatangi took Havea's long pass and fed Iongi with the sweetest of balls for the winger to waltz around Swiryn and race home.
Lavalla went close to the line but was held up short and although referee Roman Poite pulled play back for a penalty it was small beer with a 17-point deficit to claw back in the last 10 minutes.
That proved impossible when Iongi finished off his haul before Pasuka Mapakaitolo got his reward for his workhorse effort and touched down.
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