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I was thinking to a plot where the 2020 USA is attacked by the rest of the world.

Is it realistic, in our timeline, to build a story about a long lasting war (> 1 year) or the forces are too uneven ? or nuclear weapons would shortly end the whole story ?

Note that this is not a surprise attack since the tension between the USA and the rest of the world have grown constantly during the last months.
All major countries (EU, Russia, China, India, Brasil ...) declare war to the USA and they are really pissed of by the USA so no easy armistice is possible.

The big IF is the use of nuclear weapons. USA and Russia each have more than 6,000 nuclear warheads, and the rest of the world has somewhere between 1,000 and 1,500. If an all-out nuclear shoot-out begins, most of the world will become a radioactive wasteland frozen by nuclear winter. Since both sides of such a war will recognize this, it may be that neither side is willing to be the one that starts using the nuclear option, no matter how bad things go for their side. Let's assume this is the case.

If Canada is part of the "rest of the world", most troops will likely come by way of Canada, which has the longest land border to the US. The US may respond by bombing any troop transport ships (or all ships, just to be sure) heading for Canada (and Mexico and the US itself). Here, the superior airforce of the US will be an important factor. As of 2019, the US has 14,000 aircraft in service, twice the combined airforces of Russia (4,000) and China (3,000), and likely greater than the rest of the world. Adding to this are short-range missiles that can take battleships.

Naval strength will also be a factor, and here, the US isn't doing as well, with only 400 vessels (excluding patrol ships ), compared to Russia's 300 and China's 500. Russia, in particular, has many vessels designed for Arctic warfare, which is important for transporting troops to Canada.

Unless there are already lots of allied troops in Canada when the war begins, it is likely that the US will invade Canada quickly to better control the Arctic access, and the US could also quickly invade Mexico to control land access by the Central American peninsula. Both will likely lead to a lot of guerilla warfare, with the powerful Mexican drug gangs being a big factor, unless the US strikes a deal with them.

Once land access is controlled, and naval troop access is limited, the US could likely hold out for quite a long time, but the greater manpower and industrial capacity of the rest of the world will eventually overwhelm the US. Once casualties build up, there will likely be a strong call from the US population to surrender.

Should this not happen, and the US be occupied, the occupation is not likely to be peaceful. The size of the US and the number of weapons floating around would allow for large bands of guerillas striking and then dissappearing into the forests, mountains, or suburbs. In time, however, even guerilla warfare will fail, since the rest of the world could easily muster troops for a peace force equal to the entire population of the US. Satellite surveillance could also reveal guerilla strongholds, assuming that all satellites weren't destroyed during the war, leading to so much space debris that deploying new ones is unfeasible.

All this of course assumes a military situation resembling that of today. Future warfare may well be very different, with robots and drones doing most of the fighting.

Source for military statistics: GlobalFirepower.com

Yes it will last long, but not as much because of America's army size and spending and more because of logistical reasons. I'm also assuming that nukes are the alternative to surrendering, "we are about to lose, we'll nuke the world if you don't stop. Let's get back to the negotiating table with this threat ready if the negotiations fail".

Let's say the world wishes to assault America almost exclusively from the western side (So they take over Guam+Hawaii and use places like Japan, China and eastern Russia) as it's most important naval bases and stations all it's fleets there. These ships would need to spend a few months traveling the sea, then have a few months (if not less depending on the type of ship) time to operate and then have to return for servicing at a naval base. The supply chain is massive, the time active is relatively small and means that America isn't as outnumbered on the sea unless the world doesn't rotate it's fleets which wouldn't be as wise.

The first step is asking: What time frame is there before the war breaks out? This is a hypothetical scenario so bitching about "this is unlikely" isn't a thing, but we do have to consider overseas forces. If America gets time to pull out forces from South Korea and all the other countries it's stationed at then the world also gets time to plan potential war and reinforce South America and Canada before the war starts. In a sudden conflict they would lose practically all if not all their overseas forces as there is no supply line ready to transport them all the way back to America while the entire world wants to kill them. Because America is outnumbered anyway and this is a hypothetical we'll assume America pulled back it's forces and then declares war on the world with all seriousness, and the world has reasons to accept that it is a serious thing.

The best bet would be to spread the USA thin and use that numerical superiority. Ground Forces would be landed in Canada and South America. Even for USA it would be a costly endeavor to take Canada and South America in a blitz attack, time and again the attackers have always been the one's who need the most technological and manpower advantage to overcome the defenders. And that cost might simply not be worth it as they will simply increase the size of the coastline they now have to defend. A large contingent of landing ships is gathered at places like Greenland, Hawaii, Canada and South America for potential attacks.

One problem is that a D-day type landing isn't feasible. During D-day the distance the ships had to travel was comparatively small and the amount of ships available was absolutely huge. The world has a lot of ships, but almost every country including the USA itself has a great lack of solid landing ships to land large amount of troops. This would mean that the world would need to bring more supplies than soldiers on each months long trip, and the soldiers landed on the USA's coastline would need to hold out for those same months between each group of reinforcements and supply run. This would allow the USA to easily outnumber each landed group and take them out, negating the world's advantages. So these landing parties would fulfill a tactical role where they would make landfall and try to overwhelm a Naval base or airport (with help of the naval fleets ofcourse!) before the USA has time to gather enough forces to repel them, then the same landing ships would take these troops back leaving destroyed assets in their wake. Another option is to try and take large ports. Landing ships are required because ports aren't usually available. Through the use of comandeered commercial shipping and the capture of several major ports the world could make a solid landfall and open up new fronts, assuming they can locate and overwhelm less protected ports without the USA destroying them.

So a massive air-war will erupt from airbases across Canada, Eastern Russia, Greenland and the South America's. Underneath ground forces will move up, one key to capture is the Panama canal to control fleet movements more accurately, which will mean that America will subsequently try to destroy it the moment it seems under threat. The South America's won't have a large front so the USA will have a relatively easy time bottling up the world's forces there. Canada seems like it has a large front but due to the relatively low infrastructure and lots of harsh terrain the actual frontline would still be (relatively) small. To break through the world will have to rely on those landing ships to capture some ports. The best bet is to use South America as the main base as it has a relatively short range to a lot of ports compared to most options, allowing the world to quickly take control of a few ports and land a large number of troops to open up new fronts. Considering the resources available, it might even be possible to land a giant Engineering team that builds ports in a matter of months. The first goals would be island hopping to Cuba (assuming the VS took it), then using that to assault first the Bahama's and Florida. America can't defend it's islands with aircraft due to the superior number of aircraft and refueling services the world will have available, any attrition war will be worse for the USA than the world. Florida again has a relatively small frontline to push into the USA with, but in this case that could be an advantage. It makes it easier to defend and keep Florida, and then use landing ships from Florida to assault other parts of the USA coastline and start spreading it out.

So now finally the invasion is underway. Through the eastern coastlines the world can start landing more and more troops and create an increasingly large frontline. This still poses numerous problems. The USA is massive, it's land mass is varied and the patriotism and weapons available to the populace is high. Rather than try to take population centers this war would be about resources (as all modern wars between powers would be in this day and age). Control of food, energy, oil and infrastructure like highways is key, rather than trying to hold the cities. To do this you'll need to use that numerical superiority to bomb the hell out of many highways, pipelines and powerplants that are hard to reach to limit the amount of things you need to control, then overpower the USA on the remains. In WWII even Germany traveling the 400 to 500 kilometers from it's borders to Paris took 6 weeks, so crossing the USA while it is giving a lot more fight is going to take a lot longer especially since the world has supply lines across both the Pacific and Atlantic for it's troops. You can expect just 2 years of buildup of forces before the first real large-scale battles begin!

As long as nuclear weapons are not used, the US has a major advantage against the entire world, intercontinental transportation and logistics. The US has far more long-range naval and aerial transportation capability than any other country. It could use this capability to invade the Middle East (as it has several times) and cut off a large portion of the world's oil supplies. Other country's lack of trans-ocean transport would allow few forces to be required at home for self defense, allowing the Middle East to be strongly defended against Europe and Russia. This defense would only have to be sustained until oil reserves were depleted by non-US forces. Once crippling the oil supply of other nation's armies, the US could fight them at their choosing with air and armor supremacy due to the oil embargo.

Note: This idea comes from a youtube video on the subject which I cannot recall the name