They would fire theirs at us, in return knowing we would just be completely decimated as a country, our (USA) country would go into a DEFCON status, launch every ICBM/ thermonuclear weapon we have at the opposing country. Since they know they are gone, they launch all of theirs. The other countries do the same for the fun of it. Now Earth is blown up 10x over, going into a blackout, everything dies. Ice age years later, and the atmosphere everywhere is obviously toxic. Have fun!