First of All

First of All

First of all, when you say entire world, you are essentially talking about 8 countries (because there are a total of 9 sovereign states with nuclear arsenal, and that includes the US).

Let us look at the list before proceeding with your question:

• USA
• Russia
• UK
• France
• China
• India
• Pakistan
• Israel
• North Korea

Because of the way you have structured your question, I will have overlook the following crucial and critical facts:

1. UK has always been a close ally to the US (or vice versa), so the chances of UK forming an alliance with a military alliance specifically formed with the purpose of annihilating US is fairly slim. So, let us assume that there was a fairly sweet deal on the table for UK to say Yes.
2. Once again, both France and UK are also part of the NATO. So, forming an anti-US alliance looks like a long shot, but let’s ignore that as well.
3. There are just 4 countries out of these 9 who have deployed warheads in their arsenal, and other than the US, UK and France, the only one with deployed warheads is Russia. (Deployed warheads basically means warheads that are already placed on missiles, or located on active bases with operational forces)

However, there are few facts that I can not ignore:

a. 48% of the total deployed warheads globally are owned and controlled by the US (a total of 1900+ warheads). Russia controls another 40% (~1,600 warheads). France and the UK combined control less than 500 warheads.

b. US controls 45% of the total warheads available globally. Russia controls almost 50%. So the remaining 7 countries (5 of whom have zero deployed warheads) own less than 5% of the globally available warheads.

Now, given the fact that without Russia, the rest of the world has no nuclear muscle whatsoever, it is extremely unlikely that Russia will agree to this alliance unless the pot is sweetened - a lot. Think total global domination - political, economic, finances everything. And most of the countries would not even consider such a deal, let alone China.

But let us say, for the sake of argument, that this does happen and the possibility of an extremely covert and well co-ordinated nuclear attack on the US is to be a reality in the next few minutes or so, here is what will happen:

1. Though it has always been speculated that the three cities - Washington, New York and Los Angeles - are most likely to be the target for a nuclear attack, in case of a massive co-ordinated attack, the first city to be attacked would probably be Colorado Springs (on a more precise level, the target would be Peterson Airforce Base in El Paso county ). Although the population is less than even half a million, as opposed to more than 4 million in Los Angeles or more than 8.5 million in New York City, for a attack on the US to succeed, Colorado Springs would have to be the first one to go. Peterson Airforce base houses the headquarters of NORAD, and taking it out of the picture would be extremely critical as it would impede on the ability of US - to both successfully protect as well as make any offensive moves.
2. The next likely city to be targeted would of course be the city of Washington DC. Not only it houses the headquarters of the Department of Defense (The Pentagon), a surprise and stealth attack could possibly mean that the Secret Service would not get enough heads-up to move the President, the Vice-President and other important members of the Congress to the impenetrable bunkers. Since the resulting electro magnetic pulse from the detonation would disable all sorts of communication, even if any part of the US leadership escapes this annihilation, it would be extremely difficult to get the message out and they would be presumed dead. Continuity of Government protocols would immediately be implemented and a shadow government comprising of representatives from different departments and agencies would be formed.
3. All US borders would be locked down and all commercial air traffic would be grounded.
4. The military would be placed on highest alert - DEFCON1.
5. Oh. And yeah. As Daniel Holland correctly mentioned, a whole lot of grown ups would collectively shit their pants unapologetically.
6. If the source behind the attack is still unclear, all US agencies - CIA, FBI, NSA - would probably launch the biggest and most critical investigative operation of their lifetime to ascertain the source(s) behind the attack. And that would become a sitting target whatever remaining offensive capabilities US has at this moment.

Saving civilian casualties or trying to limit that would be out of the question. If this is indeed a surprise attack, there wouldn’t be enough time to co-ordinate a rescue operation, and even if it is tried, it would only result in mass hysteria and further loss of law and order and yet, no success in saving their lives. So, in all probability the government would keep it under the wraps as long as possible as they deal with the bigger issues at hand - like trying to avert the scenario till the last minute possible. A state of emergency would be declared and all press and media rights would be temporarily suspended to keep the news from getting to the public in any way.

---

Now of course it is all hypothetical and the chances of this happening are next to none. You can be sure that US invests a significant amount of energy, resources and intelligence into tracking as many of the nuclear weapons on this planet as possible - and any effort to move one to get it ready for deployment, or even ‘unusual’ activity on the bases where deployed warheads are already in place would not go unnoticed and well investigated. So, to get a drop on them is an impossible task.

[If there is a threat that US sees, if at all, it has always been from one of the terrorist organization who may get a hold of a nuclear weapon and decide to use it against the US, in which case it is likely to be a surface detonation and not a ballistic one. And that is the precise reason why they try to keep track of as much of the available nuclear arsenal as well as critical raw materials like enriched Uranium, Plutonium etc and their movement as possible. But since the terrorist scenario isn’t what we are talking about here, let us stay on point.]

But, even if, by some miracle, the JAUS forces [Joint Anti- US forces => pronounced ‘Jaws’ - for effect ;-)] are able to achieve that, the second the first missile is launched, the whole control room of NORAD will light up like a christmas tree and they would have ample time to not just evacuate the President, VP, members of congress and the supreme court justices, they would also have enough time to launch counter-measures to destroy as many of the inbounds as possible. Simultaneously, the President and the Vice Presidents will be on the hotlines with the different heads of states to resolve the situation + offer an ultimatum of a retaliatory annihilation of their country if even a single warhead is successfully detonated on US soil.

Best case scenario?

The warheads are disabled as they fall off into the pacific one by one. US never trusts anyone again, and spends the next decades fortifying its military bases around the world, using its influence, power, intimidation and money to ‘persuade’ smaller countries into ‘co-operation’. End result? Nobody wins.

Worst case scenario?

Complete annihilation of every major city you know of on this planet. So, nobody wins.

There are a lot of variables to consider if you want to know the exact effects, but almost every conceivable scenario ends badly for everyone on planet Earth. Here is a summary of what I think would happen.

1. Massive casualties in the United States from explosions and the radioactive fallout.
2. Immediate loss of life in the rest of the world caused by retaliatory strikes, radioactive fallout, worldwide economic collapse and global climate changes.
3. The disruption or destruction of electronic devices and other modern technology all around the world.
4. Decades of suffering and death all around the world.
5. Uncertain future for humanity.

Now let's look at the details.

Of the nations possessing nuclear weapons, Russia and the United States lead the pack with Russia possessing 7,300 warheads and the US owning 7,100. China, France, United Kingdom, India, Pakistan, and Israel have about between 80 and 300 warheads each. North Korea has fewer than 10 [1] .

It would be nearly impossible for all of those countries to launch all their warheads at the United States at the same time. First, of those warheads, some are retired or stockpiled. This means they are not ready to launch. This means Russia could only launch about 1,700 nukes and not 7,300.

Nuclear warheads require a delivery mechanism. The usual method of delivery involves attaching the warhead to a missile and firing said missile at the target. There are three fundamental ways of delivering a nuclear warhead to its target. Intercontinental ballistic missiles (ICBMs) are the only missiles capable of directly reaching the United States from any of the major nuclear powers. Submarines are capable of launching submarine-launched ballistic missiles (SLBMs). Strategic bombers are the third method of delivery.

Russia has 1,735 warheads deployed on 521 intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles (SLBMs), and strategic bombers. Of the United Kingdom's warheads, no more than 40 are deployed on submarines. China only has 45 missiles capable of reaching the continental United States [2] .

Going forward, I am going to have to make a lot of assumptions, estimations, and wild guesses. For instance, I estimate no more than 800 missiles and bombs with 2,000 warheads would be capable of reaching the United States. I am also assuming they will target major population centers and locations of strategic importance (e.g. military bases and ICBM launch facilities).

First, the attackers would probably detonate a few of the weapons at a high altitude well above the surface of the United States. This creates a high-altitude electromagnetic pulse (HEMP). A large device detonated at about 250 miles above Kansas [3] would affect all of the continental United States. This pulse would permanently damage electronic devices, disable our power grid and disrupt most forms of communication including cell phones, land line phones, and the internet.

The loss of life resulting from bombs hitting major cities would be catastrophic. Go toNUKEMAP if you want to estimate fatalities arising from a single weapon. Depending on the weapon yield, it would take multiple warheads to wipe out a major city. In the United States, there are around 300 cities with populations over 100,000. In theory, there are enough bombs and warheads to devastate all the main population centers.

Even this level of attack would not prevent the United States from responding with their own weapons. Mutually assured destruction (aptly abbreviated as MAD) is the strategy that guides the US's use of nuclear weapons. It is meant to deter the use of nuclear weapons by assuring the destruction of both sides. See Stanley Kubrick's Doctor Strangelove for a better understanding of this concept.

It would be nearly impossible for an enemy to completely cripple the US's ability to respond with nuclear weapons. The United States deployed nuclear arsenal is on par with Russia's. The United States has nuclear weapons on submarines, bombers, and launch facilities throughout the United States. Not all of those weapons are even in the United States. The US has military bases throughout the world, and some of those bases have nukes.

Even if the strike eliminated 75% of the United States nuclear capability, that leaves the US with hundreds of warheads. We cannot be 100% sure what targets they would hit, but I would guess major population centers and military targets in Russia at the very least. If they detonate a few of the weapons at high altitudes, the EMP could affect all of Europe and Asia.

Even after all the bombs drop, I doubt it would be enough to wipe out the United States military forces and population. About 19% of the people live in rural areas. This represents about 60 million people. The United States has over 150,000 troops deployed outside of the US .

Radioactive fallout will be a problem for years to come. The wind has the capability of dispersing radioactive fallout hundreds of miles from the blast site. Radiation will cause another wave of casualties in the short term and increase rates of cancer and health issues in the long term. There is a real chance fallout will affect Canada, Mexico and other places in North America.

We are not sure exactly how this would influence the global climate, but it would not be good. The bombs would release a lot of soot and debris into the atmosphere. Some climate models predict global temperatures could drop by 8 degrees Celsius and some regions could experience a drop of 30 degrees Celsius. To put this in perspective, global warming has caused 0.85 degrees Celsius increase in temperature since 1880 [4] and is projected to increase 1.5 to 5.4 degrees Celsius over the next 100 years. The silver lining is that this would counteract the effects of global warming.

The global economy would collapse. The global economy is complex and interconnected. As a result, something that impacts one nation impacts the entire world. The New York Stock Exchange and NASDAQ (both headquartered in New York) are the world's largest stock exchanges. The loss of these exchanges alone would wreck havoc on the global economy. Additionally, the United States will no longer be capable of trading with other nations. The United States is the largest trading partner for Mexico, Canada, China, Japan, India, Pakistan and many others. In a single day, they would lose a considerable percentage of their imports and exports.

The combination of climate change and economic collapse will likely cause mass starvation around the globe. The United States is the world's largest exporter of food. Countries like Japan that rely on food imports will not be able to feed their population. A global drop in temperature would also mean less food being grown.

There would be many secondary effects caused in the following years. As an example, the United States has thousands of satellites in orbit around the Earth. Periodic orbit corrections are needed to prevent those satellites from colliding with space debris and other satellites. It is possible that these unmaintained satellites would start a cascade of collisions that would disrupt all satellites in low-Earth orbit and make that space unusable for generations to come (see Kessler syndrome for more information). There is also the issue of the astronauts on the International Space Station. You might think they would be safe, but there is a strong possibility the high-altitude bursts could damage sensitive equipment. If they did survive, they might be stranded if no one is able to send a rescue mission.

I am not certain how this would impact global communications and the internet. The internet was designed with resilience in mind, so I am guessing it will survive in some shape or form. It is even possible Google, and similar companies could survive the initial attack since they have data centers all over the world. I expect some kind of internet to survive, but it will probably be heavily fragmented.

In my mind, the biggest uncertain is what long-term effects this would have on human society. I think humanity would survive in some form, but I am not sure what it would look like. Would the aftermath be anything like what is portrayed in movies like Mad Max or video games like Fallout? Would humanity descend into a nightmarish dystopia? Is it possible this event could unite humanity? I imagine the survivors would be motivated not to let something like this happen again.

The world essentially ends, because you can’t kill the US, Canada and Mexico faster than the US can retaliate.

If our friends, who have rather limited nuclear arsenals, attack along with the other guys, okay, maybe they live a bit longer because we don’t have MIRV’d ICBMs aimed at them.

Image: Multiple Independently-targetable Reentry Vehicle

The other unfriendly nuclear powers are not so fortunate. Also, please remember the MIRV’d ICBM submarines, carrier groups and bomber wings that will survive a first strike.

Think of the old movie WarGames. No winners, just losers in a Mutually Assured Destruction.

Image: A strange game - Airminded

Now for a problem bigger than radioactive fallout and nuclear winter.

When things calm down and if America, Canada and Mexico are no longer in the world. The poisoned, over populated, under resourced world has lost food, energy, and economy.

All in all it would most likely be an Extinction Level Event.