In 2020, humanity would be very much like as it is today, since that is only 5 years in future.
1. How our world would NOT be?
- There would be no giant robots and flying cars running around our cities.
- There would be no nanobots fabricating graphene-based complex nano-machines in industrial scale.
- There would be no cyborgs with broad-band telepathic quantum-digital communication.
- Genetic engineered mutants will not be warring to displace humans.
- There would be no AI-based coordinated and efficient all-Earth government.
- There would be no superintelligent beach crabs who calls Earth their homeworld projecting and building a Dyson sphere or a Death star.
2. How our world would be?
The western world will be fighting jihadists in Middle East. UN will be paralyzed and ineffective due to Russia's and China's successive vetos to anything that could have some real effect in practice.
People will be talking about global warming. People who earn money by burning or selling fossil fuels will deny it.
There will be some people dying from malaria in some poor and disorganized countries.
There will be some tensions and hostilities between Israel and Palestine.
Obama's successor will be thinking about something to do with Cuba (not something military). Specially if he/she happens to be from the republican party.
Apple would be announcing their new iPhone 8.
Facebook will be launching some new services to compete with Google Alphabet.
Microsoft would be receiving a lot of complaints about their new and buggy Windows 12 and would be preparing a large patch called Windows 12.1. Bing's social-cloud-network integration in the OS raises a lot of complaints about privacy and anti-trust commercial practices, but it will reveal to be a very useful feature to allow botnets to download and install malwares in user's PCs.
NSA will be illegally tracking and monitoring your traffic in porn suspicious sites in order to find for interesting nudity criminal evidence.
Thanks to many technological breakthroughs, solar and eolic energy are indeed the future and they are very cheap in 2020 - long life to our green energy! But instead, governments and corporations would still prefer to burn oil to keep the money in the hands that it always used to be. Screw up those nasty green ecoterrorists and their stupid solar and eolic technologies!
3. What would happen when the aliens arrive in 2020?
People will suspect that the thing is some sort of secret spacecraft developed in Area 51. US government will (as always) deny. Nobody will initially take US's deny seriously. And this is very ironic, since for tens of years, people who had seen secret airplanes/spacecraft tought that they were alien starships. Now that they are seeing actual alien starships, they will think that they are secret airplanes/spacecraft.
North Korea's government will tell their people that it is a North Korean spacecraft proposed and built by Kim Jong-Un himself and that this proves that North Korea has a tech level much more advanced than the rest of the world.
Islamic fundamentalists will say that the UFO is a sign of Allah's power. Some Christians and Muslims sects would state that it is Jesus coming back in a spaceship directly from God. Some other Christians and Muslims sects would say that it is the antichrist instead.
UN tries to approve some meaningless bureaucratic resolution about the UFO incident. After some months of talking and debating, it is vetoed by China and Russia just for the fun of vetoing.
US will entitle themselves as "the world" and try to establish contact with the aliens. If US concludes that the aliens are friendly, then they came here obviously because they were interested in learning more about the fabulous America's way of life. If they are enemies, it is because they are communists and came to destroy America's liberty. As always, US will forget that there are other countries in the world.
Most countries in Europa, Africa and Latin America will just not give a single [insert a bad word here] about it. Oh, we are being visited by aliens, but who cares, let's just continuing watching stupid commercial TV shows and sharing stupid memes on internet.
Some visionary prophet will start a new religion to worship the newcomers (which he will state that came from Nibiru and Hercolubus) as the returning god astronauts from ancient Egypt and will announce that this is the start of the war against the illuminati. Also, he will show a "proof" that the Mayans miscalculated the world final date by 12 years and that it will end in year 2024. Millions of people would faithful join the religion (for a small monthly fee, of course). The visionary prophet will then quickly get some millions of dollars in his account, some mansions, yachts, airplanes, Ferraris and Lamborghinis. He will always be seen using a Rolex watch and his multiple top-model wives will dress clothes made from gold and diamonds in Caribbean beaches. However, he will always make clear that all his money is for charity, for African hungry child and for saving humanity in those end days and that he never took a single dollar from his followers for himself.
Scientology church, to not get much behind other UFOs religions, publishes a new OT level about Xenu's past plans on planet Teegeeack and link it to the UFOs. But this information would be held secretly and would be released only for those people who had cleansed themselves enough (by providing them a lot of cash), except that it will leak in the internet in the first 5 minutes. Anyway, they would still aggressively insist in trying to sue and persecute anyone that publishes their BS and state that unprepared people who reads it will die from pneumonia.
Then, the aliens impatient and deceived by Earth's people disorganization and stupidly, secretly perform some changes in Earth's society in order to match the OP's expectations. Otherwise, the attack would be too easy for them and they would not have any fun from it.
After the aliens secretly changes Earth's society in some way, all the governments in the world suddenly agrees to coordinate people about investigating the aliens. They are proud that they quickly agreed that undoubtedly the aliens seems to be scouting Earth and that they will attack. However, in truth, the aliens deliberately gave the information to the humans that they are scouting and that they will attack, and did their operations in blatantly open and obvious ways, including by uploading their full and detailed plans in Youtube already properly translated in a lot of human languages. Otherwise, the stupid humans would probably never figure it out what they were doing.
An organized world government finally forms, and perceiving that their time is short, start to heavily and hastily prepare for the war. See part 8 below for more details on some things that they might do.
4. Could humans war against the aliens?
Probably no!
Let's suppose that we choose to attack their ships:
They have enough technology to coordinate a lot of spaceships travelling through the galaxy, and who knows how many spaceships are there?
If they are sending a lot of scouting ships that travelled though the galaxy, what would be their battleships?
They surely have a tech level much more beyond of what we had ever dreamt about, in order to be able to achieve spacetravel.
If we could ever harm one of their scouts, an alien counter-attack would probably simply obliterate the humanity, perhaps in a few minutes. Again, they have enough tech for star-travel.
Even if they choose to use H bombs just as the same as humans developed during the cold war and in the same number that humans developed, and then land to drop those and explode 'em, humanity as we know it would be gone and we would not have any way to defend against that. We already have enough technology to wipe out ourselves even nowadays (as 2015), so why would a space-travelling alien civilization not be able to easily do it? Also, they probably would have something much more advanced than that if they want to attack us.
So, the conclusion is that attacking them would be like trying to use sticks and stones to defeat an horde of flying Apache helicopters equipped with missiles. No people with at least half a brain seriously thinking about it for 10 minutes would ever conclude that we have even a tiny chance in succeed on driving them away by war.
Earthlings would likely try to insist in communicating. But, goddammit, no matter what, we got no answer, at least no answer that we actually could understand as such. They simply don't seems to want to talk to us. Perhaps they don't want to talk to humanity by the same reason that you don't want to talk with the ants that are nesting in your garage. We are just too primitive.
5. Why could the aliens be here?
Ok, if the aliens would like to just destroy this worthless peace of rock and its stupid Moon, or even simply wipe out humanity from the third planet orbiting the Sun, we probably have no chance against them. So, if this is their ultimate purpose, we are probably doomed.
Maybe they want a new planet to live and are checking if Earth is suitable? If this is the case, we should welcome them and hope that they don't think about exterminating us. But it would help a lot if they actually wanted to communicate (or perhaps, they are already communicating, by uploading their full detailed plans in Youtube).
Maybe they want some mineral, or biomass, or water, or something else from Earth. If so, we could live our lives providing what they want. If they are hostile, but don't want to just kill us, we may be fine by accepting slavery. Of course, this would only be viable if they actually communicate.
Maybe they just will take out of this planet or put it here, or do whatever they want to do undisturbed, without telling anything to us. We could watch them in hope of learning something. Trying to attack or disturb them would probably be useless. They would either don't care and just continue whatever is they work, or they would retaliate.
6. We really need to war against them. How could we have some chance?
If we want to war with them anyway, we could have some chance against them if:
They are severely depleted of resources. And this could be one of the reasons of why they are coming. If the resource is something simple, they can easily go to Mars or the Moon and get it, but if they are solely on Earth, this means that they need something that is abundant only on Earth. Possibly something developed by life activity or by humans theirselves. What we could do to try to resist is try to intercept them as soon as possibly, use anything that we could do to harm them (like missiles), and try to hide or destroy whatever they want to take. We must put our hopes that their resource-depletion makes them somewhat unable to react or defend themselves properly.
They are severely depleted of personnel. Maybe the aliens are just a small couple of pirates or refugees fleeing from their planet coming to Earth in order to hide from their enemies. In this case, if we agree that they are unwelcome, for example, if they are a small number of space locusts that devours all the resources of habitable planets and then just leave for another planet leaving an inhospitable wasteland behind, we should try to locate their members and kill them. Every kill in this scenario is a win even if we lose 10000 humans for each alien kill.
If they are not severely depleted on either resources or personnel, and we could not just adapt to live with them at Earth, then we are doomed. Our only hopes is that they are severely depleted on either those, and even if that is the case, we still have few chances against them.
7. I spotted an alien and want to kill him. What to do?
If the alien is inside a flying spaceship, launching a missile might be doable. They could counter attack, but if they are resource depleted, they might instead choose to flee. They might be able to dodge a missile easily, so lasers might be a better option. Each destroyed spaceship is a score for the humans.
If they are just walking with some spacesuit over the land, we could attack them with lasers, guns, arrows, stones, whatever. If their spacesuit is sufficiently damaged, this could mean their death. However, they are likely to be armoured, so damaging their spacesuit might be very hard, specially if it is done from some virtually alien indestructible material.
An alien walking around could be trapped. We could try to imprison them inside cages or crush them with rocks (probably by strategically positioning dynamites and ambushing them).
If the alien is a robot, EMP pulses might also work in addition to explosives, lasers and traps. If they are not armoured for that and unable to dodge or flee, of course. Also, if they are resource depleted, it is possibly that their robots are fragile and lightly armoured.
Grabbing parts of destroyed spaceships, destroyed robots and dead aliens is very important. It becomes much better if you can capture and safely hold working spaceships and robots or living aliens. The most that we can collect from them in order to learn what they are and how they behave, the better. Kudos, if you can hack their spaceships and robots to make them work for you, or convince some alien to change side and work for the humans instead.
8. What to do to defend/resist them?
If they decide to just blow up this planet, of course, there is nothing to do.
But if they are hostile, but don't want to chase down the humans (or not spend much time and energy in that), other than attacking them with the tactics in the part 7 above (if any of them works), we could:
Avoid them. If they just want to do whatever they want to do undisturbed, we could just let them do. Except if those activities are too harmful and untolerable for us, of course.
Live inside bunkers, mountains, forests, ships on the oceans and any place far away from the aliens.
Learn how to plant yourself, store food and seeds in safe hidden places. If possible, plant in vases that could be quickly moved from one side to the other. Prefer to produce foods that are nutritive, cheap and easy to grow.
Anticipate the enemy moves, and take everything valuable that they might want to use or that would be destroyed. This includes, for example, crops, water and machinery.
Mass-produce solar cells in a size usable by individual bunkers and that are sufficiently cheap, easy to produce and easy to install. We should do that when we still can.
Try to sabotage or steal whatever they produce here.
Invest heavily in seawater desalinization in order to provide people who lives in the sea or near the sea a reliable way to get drinkable water. We must invest on that as quickly as possible before our industry collapses or degrade to a level where we are unable to do that anymore.
For people living inland, provide ways to get drinkable water from rainwater or from rivers.
Invest heavily on communications. If they don't attack our satellites, we should launch as many as possible to ensure efficient communications. Otherwise, we should build the most that we can of ground, underground and underwater communications cables. Coordinating radio transmissions and band-width is also critical. Mass-producing all the equipment for that is also very important. And very important, deploy that f***ing IPv6 for once and all, we already had enough of IPv4.
Invest heavily in recycling materials. Avoiding wasting anything that could be useful.
Invest heavily in projecting machines that are cheap, easily producible and are easy to fix or replace parts if broken or damaged.
Invest heavily in war education. Everyone, including child and elderly people, need to be trained on how to act quickly and correctly in order to don't help the enemy, nor be a victim of them and to attack or sabotage them. The training also helps to improve resistance coordination. Collect as much information as possible from the aliens and teach whatever not deemed too secret you can. A lot of people need to know how to attack the aliens and also teach that to the others. Further, the education should be bended in a way to not tolerate traitors and to make people suspect on any behaviours that a potential traitor could have.
In other words, what humanity need to do to defend itself is similar to what a nation being invaded by an enemy army and trying to resist that does in a war. The economy and people behaviour change directly in accord to that. Think in what was Soviet Russia, UK and Nazi Germany during WWII. The economy of those three countries turned almost entirely into a war economy, trying to frenetically outperform the enemy as quickly as possible and harm it at the same time. Parts of the economy deemed as superfluous suffered or even stopped during wartime.