Answer:
First, nuclear weapons create very large explosions capable of destroying entire cities, and secondly, they also create large clouds of radioactive material known as fallout, which would poison the entire world in the event of a large or full scale nuclear war (as compared to WW II when only two relatively small atomic bombs were used against Japan). There are also other possible problems resulting from a nuclear war, such as what is known as nuclear winter; the amount of dust and smoke in the atmosphere would be so great as to block out the sunlight and plunge the world into extreme and prolonged cold. It is doubtful that the human race (and many other species, perhaps most species) could survive a nuclear war.