USA captain AJ Mac-Ginty was almost in tears after the Eagles missed out on the final qualification place to Portugal on points difference.
Portugal secured the draw they needed to make it to a second World Cup when Samuel Marques' penalty levelled the scores at 16-16 with the clock in the red to win the four-team final qualifying tournament in Dubai and book their place in Pool C alongside Wales, Australia, Fiji and Georgia.
MacGinty and Marques matched each other kick for kick, converting each of their team's tries as well as slotting three penalties.
But it wasn't enough for the USA who miss out on appearing at a Rugby World Cup for only the second time in history.
“It's gutting for how hard the boys have worked but that's the nature of the beast,” said Bristol fly-half MacGinty.
“It hurts even more when you know how much you have put in and how much you have sacrificed. I am extremely proud of the effort the lads put in but, unfortunately, it wasn't to be.
“It's so disheartening how this has finished. A lot of the boys broke down in tears and I am almost there myself.”
For Portugal captain, centre Tomas Appleton, there was only delight at what lays ahead.
“There are more than one million Portuguese people in Paris. To play at a Rugby World Cup in France will be amazing,” he said.
It is only the second time the USA have missed out on qualification in 10 World Cups.
The Eagles' failure is a blow for the country, who will host the 2031 tournament and the 2033 women's event.
Portugal scored the only try of the first half after eight minutes when wing Raffaele Storti crossed. Marques and MacGinty traded kicks and Saracens hooker Kapeli Pifeleti scored a try in the second half to put USA ahead.
But Portugal earned a penalty a the death which Marques landed after Jeronimo Portela's drop-kick had hit the post seconds earlier.