In 1945, we used the atomic bomb twice in order to end World War II. Had we not used it, almost certainly hundreds of thousands more people would have lost their lives needlessly, including many American soldiers. The bombs were dropped from airplanes. A necessary precondition for this attack was to first secure the airspace over Japan so that Japanese air defenses could not shoot down the bombers. But before American forces could secure Japanese air space, they first had to get close to the Japanese coastline. This effort, alone, cost us hundreds of thousands of lives.
The next time a nuclear warhead gets used, things will be very different:
The United States will no longer be the only nation on earth possessing nuclear weapons.
Among the 202 sovereign countries of the world, 30 have either begun or completed their own development of nuclear weapons or have purchased such weapons from some other state. Of these nations, 10 are known to possess nuclear fission warheads, and 6 are known to possess nuclear fusion warheads. Only 188 countries have signed the Nuclear Non-Proliferation Treaty; of these, three, including Iran, are known to have broken the treaty.
As the number of nuclear-armed countries goes up, it becomes more difficult for us to counter them all. Furthermore, should we be struck by a nuclear missile, it will be correspondingly difficult to ascertain who launched it. (During the 1950's, that calculation was easy: only the Soviet Union could strike us.)
The preferred delivery vehicles will no longer be airplanes.
Securing the airspace over a defending nation is costly in lives to the attacking nation. A nuclear missile minimizes the loss of life among the attackers. Therefore, in today's world, the inexorable logic of combat almost insures that nuclear missiles will be used.
The logic of combat also dictates that expensive weapons will be used only where they will inflict the most serious harm upon the enemy. Nuclear missiles are very expensive. Therefore, they are mostly designed to be city-killers. Launching a nuclear missile at open farmland makes no sense. The only notable exception to this rule is a nuclear missile specifically designed to break open a hardened bunker. Such redoubts are used to hide the defender's most valuable assets; typically, these are the defender's own nuclear missiles and nuclear production facilities. These tend to be hidden underground in rural areas.
The devastation will be greater.
The yield of the Hiroshima bomb was equivalent to the explosive force of 15 thousand tons of TNT. Today, a large fusion warhead delivers the equivalent of 50 million tons of TNT.
We will be much more vulnerable.
During the 1960's, disabling the United States would have required the attacker to launch thousands of nuclear missiles. Today, because so many of our vital networks pass through New York and Washington, two missiles may be all that is needed to cripple us. The roads, railways, air routes, and telecommunication lines that pass through these key cities will be disrupted. Essential goods and services, most especially food, will take much longer to reach us.