IBM's Sequoia has taken the top spot on the list of the world's fastest supercomputers for the US.
The newly installed system trumped Japan's K Computer made by Fujitsu which fell to second place.
It is the first time the US can claim pole position since it was beaten by China two years ago.
Sequoia will be used to carry out simulations to help extend the life of aging nuclear weapons, avoiding the need for real-world underground tests.
It is installed at the US Department of Energy's Lawrence Livermore National Laboratory in California.
While Sequoia may be the fastest, the underlying computing capabilities it provides give us increased confidence in the nation's nuclear deterrent, said National Nuclear Security Administration (NNSA) administrator Thomas D'Agostino.
Sequoia also represents continued American leadership in high performance computing.
Adding processors
The computer is capable of calculating in one hour what otherwise would take 6.7 billion people using hand calculators 320 years to complete if they worked non-stop.
Although the US's efforts helped secure it the lead, its overall tally of three computers in the top 10 was worse than six months ago when it had five.
China and Germany both have two supercomputers, while Japan, France and Italy have one.
But IBM proved to be the leading manufacturer claiming five out of the top 10 spots.
David Turek, vice president of deep computing at the firm, told the BBC his company had been preparing to retake the top spot for two years.
Substantial planning went into this. We knew the day would come.
Sequoia is 1.55 times faster than the Fujitsu model, and uses over 1.5 million processors.
In comparison the Japanese model has less than half the number of CPUs (central processing units).
Energy efficient
The IBM supercomputer is also more energy efficient than the Fujitsu model.
Sequoia consumes 7.9 megawatts compared to the K computer which uses 12.6 megawatts.
Mr Turek described Sequoia as the pinnacle of energy efficiency and said the reaction had been very enthusiastic.
Government laboratories in Europe have already expressed interest, he said.
The list is published every six months by German Professor Hans Meuer and US-based Professor Jack Dongarra.
Prof Dongarra told the BBC it was unlikely that another manufacturer would overtake IBM in the next year.
Sequoia is very impressive, he said.
273,930 times faster
The first computer to take the top position on the list was the CM-5/1024 in 1993, designed by Thinking Machines.
According to Prof Dongarra, Sequoia is 273,930 times faster.
A calculation that took three full days to compute on the Thinking Machines in 1993 today can be done in less than one second on the Sequoia, he said.
