Oddly enough, the military will be the first customer for a one petaflop computer for tactical and strategic scenarios/simulations, which will no doubt include WWIII type situations.
For the near future, it will almost certainly be used to comply with the moratorium on nuclear testing.
Over the last ~20 years many of the biggest supercomputers on the TOP500 list were purchased to run simulations of the aging US nuclear stockpile to answer maintenance questions that formerly required active testing.
