An excellent question indeed! How would we fight an very advanced race that is invading? The following is from interestingengineering.com and written by Christopher McFadden:

"First contact with aliens is a common theme in many science fiction stories. Often depicted as either the most remarkable event in human history or its end, it would undoubtedly change everything we have ever thought about existence forever.

But, should things turn nasty, what could we really do to evade extinction? Let's take a look.

Would extraterrestrials be aggressive?
It is probably worth exploring if we actually have anything to fear before we get into the nitty-gritty about potential defensive strategies for our planet. While we can only know for sure if an alien species would be aggressive by actually meeting them, we can make some educated guesses about how such an encounter may turn out.

Some of the greatest scientific minds, like Stephen Hawking, have famously warned about not announcing our existence to the universe. He believes that bumping into extraterrestrial life would be devastating for our species and planet.

"If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn’t turn out very well for the Native Americans," Hawkings said in a 2010 documentary for the Discovery Channel.

"A civilization reading one of our messages could be billions of years ahead of us. If so, they will be vastly more powerful, and may not see us as any more valuable than we see bacteria," he later qualified.

Whether intentionally or by accident, an alien species may wipe us out, or perhaps worse, consider us as a resource to be harvested. This, for example, is the premise of the First Formic War Trilogy in Ender's Game series of novels written by Orson Scott Card and Aaron Johnston.

In this series, the ant-like aliens called Formics do not consider human beings sentient since they lack a "hive mind". To this end, they begin to prepare Earth for a new colony. To this end, they mount an invasion but are defeated through various interesting tactics and extreme sacrifice.

While this is obviously fantasy, it does raise some interesting questions.

For example, what would aliens look like? Many thinkers in this field believe that our first contact with aliens will be with robotic probes or drones rather than living creatures. Similar to how we do not risk human lives on more extended space exploration missions, aliens may adopt a similar strategy.

This might be for a variety of reasons, but the time required for long journeys and investment in life support systems are chief among them. Yet other experts also suggest that such robots could be the aliens themselves.

"Because of the limits of biology and flesh-and-blood brains," Steven J. Dick, an astronomer and ex-chief historian for NASA has argued. "Cultural evolution will eventually result in methods for improving intelligence beyond those biological limits," he added.

Robotic aliens are also more likely to make such long journeys, as they will be far more durable than any kind of life we currently know.

However such aliens look, many experts have also warned that they will likely be unpredictable. If they are millions or even billions of years ahead of us in development, the outcome of meeting them may not be in our favor.

It is quite likely they'd be capable of some aggression, as this kind of behavior is generally favored by natural selection, at least here on Earth. After all, a species that cannot fight to defend itself or hunt is less likely to develop the capacity for exploring the unknown and taking the risks of space travel.

On Earth, at least, predatory animals tend to be the ones who have developed problem-solving abilities. The same is probably may well be true for alien life.

It may also be the case that aliens, like our own species, have developed the capability for both extreme violence and peaceful compromise.

"We have good reason to believe that aggressive instincts will be present in extraterrestrials," astrobiologist Pushkar Ganesh Vaidya has written. "To what extent alien life can curb their aggressive instincts (or else they will possibly self-destruct) is anybody’s guess," he added.

There is also the possibility that humans could unintentionally spark aggressive actions from aliens. Since such a meeting would be incredibly tense, the chances of events spiraling out of control are quite likely.

This sort of scenario has been covered in various sci-fi works for many decades now, but we will never really know the outcome of such an event until it happens. enders

How could we fight off an alien invasion?
So, assuming that a visiting alien race may have less than peaceful intentions, what, if anything, could we actually do about it?

Let's take a look at some of the current thoughts on the subject.

1. It would be nice to have some warning of an impending attack

From films like "Independence Day 2" to "Starship Troopers", many science fiction franchises suggest that in the future, our species will have spent some time building up an early warning system of some kind to defend our planet. In real life, such a system(s) could be incredibly useful for giving us as much time as possible to prepare for what many thinkers consider an inevitable clash of species.

While we have many space observation telescopes on Earth and in orbit, most of these are not explicitly designed to look for alien craft. However, so long as alien spacecraft don't defy the laws of physics as we understand them, they could have some use for this task.

For example, NASA's Wide-field Infrared Survey Explorer (WISE) space telescope is probably the best bet at present. Of course, assuming that an alien ship is large and slow enough to be picked up and tracked.

Like the Search for Extraterrestrial Intelligence, or SETI, other programs are specifically designed for hunting for potential alien communications. But, most efforts scan only a tiny portion of the night sky at any one time, and we would need to be very lucky to detect an incoming ship or even fleet using these. Assuming that aliens would be broadcasting as they approach a potentially inhabited planet.

However, we do have some infrastructure on Earth tasked explicitly with hunting for potential threats to our planet. For example, through the Planetary Defense Coordination Office, NASA constantly monitors the sky for incoming objects, like asteroids, that could prove dangerous for life on Earth.

While NASA and similar organizations are primarily searching for things like near-Earth objects (NEOs), such systems could also look for incoming spacecraft.

Thanks to existing systems like this, we've had some interesting false alarms for potential visiting alien craft over the last few years, like 'Oumuamua.

But, we may have some better systems in the works as well.

A concept called the Universal Robotic Battle Cosmic Platform (URBOCOP) might be just the ticket. According to its designers, the Moscow-based International Expert Society on Space Threat Defense, such a system would be completely autonomous and could simultaneously identify, track, and classify potential space threats to our planet.

URBOCOP would be an armed, unpiloted space station with onboard weapons capable of destroying both natural and human-made objects threatening Earth.

Using a fully automated system, the international platform could monitor threats such as asteroids and comets, space debris, ballistic missiles, and, yes, perhaps even alien ships. It could even be outfitted with its own planetary defense weapons, too like a silo of nukes or kinetic energy weapons like railguns.

However, since such a platform could also be used to potentially target things on Earth, we'd have to be very careful with any systems used to automate it to ensure they are free from human bias. It would also need to be "hackproof" from other humans and aliens.

But, even with all things going to plan and assuming that we are given enough time to prepare, what can we do next?

2. Don't let them land, nuke them in space

Once we've detected an approaching spaceship, assuming the aliens are known to be hostile (which may be impossible to determine in advance of their arrival), what actions could be taken to stop them? Since an actual full-scale invasion would likely get very messy, the best option for us would be to stop them dead, so to speak, before they get to us.

Nuking them might be an option. But would this actually work?

Expert opinion is, unsurprisingly, divided on this subject. Some believe that alien spacecraft would likely be designed and built to be as light as possible. This may make them susceptible to nuclear strikes — if we can actually hit the ship.

However, like plans to defend Earth from asteroids, we'd likely need to penetrate the ship and detonate bombs inside it rather than impact them on the surface. The latter would probably have minimal effect on a large and tough-enough spaceship especially as any deep-space going craft would need considerable shielding.

Believe it or not, researchers have considered plans for spacecraft capable of doing this today. One example is called the Hypervelocity Asteroid Intercept Vehicle, or HAIV. This spacecraft is designed to carry a nuclear weapon to an approaching asteroid and blow it up before it becomes a threat to Earth.

The idea is that the craft approaches an asteroid, bores a hole through its exterior, plants a nuke, and then blows the thing to pieces in short order - a bit like in the movie Armageddon. While currently very much in the conceptual phase, crafts like HAIV are not beyond our technological capabilities to build today.

It should also be possible to weaponize such a device to intercept and potentially knock out an alien spaceship. But, once again, we would need to ensure that this craft could actually bore a hole through an alien spaceship's hull.

Or, for that matter, even stand a chance of getting near enough to do so without itself being blown up.

3. If they landed, could we just shoot them?

Should our most devastating weapons, nukes, fail to stop them in space, and they launch an invasion, what can we do? Would small arms, for example, be any use at all?

Once again, opinion is mixed on the subject. But, if defensive weapons like energy shields are a scientific impossibility, as some believe, then could weapons such as firearms hurt them?

Possibly not.

Let's consider the fact that an alien race would need some very sophisticated technology to travel the vast distances of space unscathed. It is likely they have developed some very resilient materials. Their spacecraft, for example, would need to be able to survive multiple impacts from fast-moving, high-energy objects like micrometeorites.

Such 'armor' plating, or rather the material it is made of, could be adapted for body armor (or combat drones) that would have little trouble dealing with a speeding bullet. And this is not science fiction. Existing human spacecraft are often armored to protect them against speeding micrometeorites and "space junk" in a similar fashion. Other materials like nanostructured materials currently in development also show some promise for future near-impenetrable protection.

If such armor can stop speeding micrometeorites, it should also have little problems against weapons like bullets, knives, swords, spears, giant stones, etc.

With their higher mass and energy, artillery shells may prove effective on individual aliens but would probably be ineffectual against their vehicles.

4. Are there any secret weapons we could use against aliens?

If aliens ever attacked us, is there any possibility the world's military powers have something secret up their sleeves to save us? After all, this is often what happens in the movies. While such programs are of utmost secrecy by their very nature, most of the advanced weapons we know about today were also once hidden from view.

The Pentagon’s Defense Advanced Research Projects Agency (DARPA), for example, is famed for its secret weapons research. Defense contractors are another potential source of some futuristic tech, too.

We can only really speculate about what is currently in development, but some rumors may have some truth to them, and others that we do know something about, of not everything.

One example is the High Energy Liquid Laser Defense System, or HELLADS. While its name is less than inspiring, this directed energy weapon is not something to be taken lightly.

Currently, in development by DARPA, this system would utilize high-energy lasers capable of targeting, tracking, and ultimately destroying targets. At present, the lasers under development (as far as we know) are too heavy to mount on an aircraft, but plans are afoot to miniaturize a 150-kilowatt variant in the future.

Whether such a weapon would have any utility against alien craft can only be speculated on.

DARPA might also have something in the works for super speedy and maneuverable aircraft that could prove pivotal. Called the Falcon HTV-2, this is an experimental hypersonic, uncrewed, rocket-launched aerial vehicle.

According to DARPA, this craft will be able to reach Mach 20 and appears to be primarily designed as a reconnaissance craft. With some additional development, it may be possible to actually weaponize such a vehicle and unleash it on some unsuspecting alien forces.

Other potential experimental weapons include the Magneto Hydrodynamic Explosive Munitions. Called MAHEM for short, these projectiles use “compressed magnetic flux generator (CMFG)-driven magneto hydrodynamically formed metal jets.”

While further technical information is scarce on the ground (for obvious reasons), these weapons appear to work by using a magnetic field to push molten metal into a target. Again, heaven knows if this would do any actual damage to an alien or alien craft, but it would certainly be worth a punt.

5. So, if we can't nuke them or shoot them, what's left?

If our mightiest weapons fail us, what do we have left? Probably only enough time to make peace with our makers.

But seriously, what would be our final line of defense?

While we, as species, are incredibly adept at fighting conflicts here on Earth, our technology and tactics would probably be woefully inadequate for resisting an alien attack of this kind.

Such a conflict would likely be short, brutal, and devastating. In fact, some experts, such as Annie Simon (a biology professor who was an adviser on The X-Files), believe such an encounter would be "like the Roman Empire fighting the US military today."
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End of Part one.

Reference: https://interestingengineering.com/how-to-stop-alien-invasion