Of course there are those who would like it if war was abolished. While that would be a nice idea, it seems to be a bit impractical. The old bromide is that people who beat their swords into plowshares tend to be killed by people who keep their swords intact, though sometimes they get lucky and are merely enslaved instead of killed. There are additional reasons for war if the others are aliens.
So it appears that war will be with the human race for the forseeable future, or can be avoided by becoming not human anymore.
Just to be complete, there are some odd activities that are arguably "warfare" but they sure don't look like it. These turn up especially in science fiction, which often is concerned with pushing the envelope and questioning basic assuptions.
I am not going to go into them in much detail, because they are mostly invented out of a whole cloth by the science fiction author. Which makes it very hard for me to generalize. Not to mention the fact that the vast majority of the readers of this website are not interested, they are here for the space navy and interstellar grunts.
Unconventional warfare, the opposite of conventional warfare, is an attempt to achieve military victory through acquiescence, capitulation, or clandestine support for one side of an existing conflict.
Divide and Conquer
That old gag of "clandestine support for one side of an existing conflict" is a classic, and it never gets old. Economical, relatively risk-free, what's not to like?
For this to work you need an existing conflict, which means the operating theater must be balkanized. Make covert contact or send your agents into a couple of the most powerful nations and encourage them to attack each other. The loser of the war will be in no condition to resist your invasion, and the winner will be vastly weakened. The idea is "Let's you and him fight." In The Art of War, Sun Tzu called this "attacking alliances." A common colloquialism is "play both ends against the middle".
A balkanized planet is just full of flaws and vulnerabilities for an invader to take advantage of. The invaders can try to covertly inflame old hatreds and grievances, corrupt a nation into doing the invader's bidding by dangling riches or valuable alien technology in front of their nose, frame one nation with something it didn't actually do, the possibilities are endless.
Isaac Kuo points out that this also has implications for the invaders. If the invaders do not have enough troops to conquer an entire planet, but only enough for one nation, the dynamic shifts. As he puts it:
This is a variant on the old joke "I do not have to run faster than the lion, I just have to run faster than you."
Isaac also mentions that if the various balkanized nations hate each other enough, when the invaders attack one nation, that nation's enemies might actually pay the invaders in gratitude.
A related concept is "Proxy War." In the prior strategy, the operating word is "clandestine", your opponent is unaware you are involved. Also, your opponent is one of the factions in the conflict. In a proxy war, your opponent is not directly involved, and they know quite well you are involved. The actual combatants may or may not know.
During the Cold War, proxy wars were standard operating procedure. Due to the fact that if the United States and the Soviet Union fought directly, sooner or later nuclear war would erupt. It was safer to damage your opponents interests by using a proxy.
Another potent type of unconventional warfare is Economic Warfare. Cutting a planet off from interstellar trade could trigger a major recession and may even crash the entire planetary economy.
Traditionally, most wars are caused by disputes over resources. That is, the cause boils down to "two monkeys and one banana."
Most of the remaining wars are caused by the fact that human beings tend to be obnoxious self-centered arrogant authoritarian tribalistic creatures. That is, the cause boils down to "two monkeys, both of which want to be boss." You can identify these types of conflicts because there are no resources being squabbled over.
But please understand that bombing a planet back into the stone age is something that makes more sense in simplistic space operas, not in realpolitik. Sadly saturation bombing does make sense in a religious war. By which I mean it is an irrational senseless act, but an act that could plausibly happen in the real world.
Ken does have a good point. The motivation of the invaders puts limits on the allowed invasion techniques. If the invaders want slaves, it is counterproductive to kill every living thing on the defending planet. If the invaders want real estate, it is counterproductive to dust the planet with enough radioactive material to render it uninhabitable for the next ten thousand years. And so on.
The lack of a logical reason for invasion is up to the author to devise a solution for. Some of the motivational questions can be side-stepped by assuming the invasion is not an alien one, but instead a hypothetical human interstellar empire attempting to invade a human colony world. The motivation of the empire can be something stupidly human like "gotta collect 'em all!". This is actually the motivation in Larry Niven and Jerry Pournelle's The Mote In God's Eye. In that novel, there once was a loosely allied human interstellar empire that collapsed in a bloody secession war. The new imperium rose from the ashes, grimly determined that such wars will not happen ever again, and all human worlds must be incorporated into the empire with no exceptions.
A war of aggression is a war for conquest or gain. The former is driven by a national leader who wants to expand the territoral limits of their empire because Bigger is Better. Nyah-nyah, mine is bigger than yours! Alexander the Great's fixation in other words.
Another motivation is to obtain more real estate for your people to live and expand on. Once your army has kicked out the current inhabitants. The clinical word is "Lebensraum". This does not apply to land being occupied by extreme alien species, since they generally live on hell-hole planets you can't live on anyway.
A third motivation is Empire Security. This is when the empire wants to be surrounded by a safe armor composed of buffer states. These are small empires lying between two rival or potentially hostile greater powers, which by its sheer existence is thought to prevent conflict between them.
A War of Independence is when a territory under the jurisdiction of a state gets fed up with taxation without representation and other oppression and declares its independence. The owning state almost invariably disagrees, and the war starts.
In science fiction the "territory" is usually some species of planetary or space colony. But not always.
(ed note: A civil war, on the other hand, is between two organized groups inside a single state.)
Many authors have been inspired by the American Revolutionary War and similar revolts. History repeats itself. So authors figure if Mars (for instance) is colonized, then Terra starts acting like King George, history will repeat with Mars emailing several megabytes worth of Declaration of Independence to Terra and starting the training of Martian minutemen.
TV Tropes calls it the The War of Earthly Aggression (though their sarcastic name is a riff off one of the titles of the American Civil War, not the American Revolutionary War). Related TV Tropes are The Revolution Will Not Be Vilified, The Revolution Will Not Be Civilized, La Résistance, and Les Collaborateurs.
In the section on elemental bottlenecks I point out that phosphorus and nitrogen are vital for plants, animal, and people; but there are no rich sources in the solar system except for Terra. This is not an insurmountable problem for a spacecraft or space station. But it is a major catastrophe for an extra-terran solar system colony. Your supply of new baby colonials is limited to your supply of phosphorus.
This could be a large club that the government of Terra waves at the extraterrestrial colonies, if they start making noises about rebelling from Terra's oppressive control. If the Martian colonials start complaining about "no taxation without representation", Terra will respond with "You are receiving a nice steady supply of phosphorus. It would be a shame if anything happened to it." Naturally the Martian Revolutionary War might be kicked off by the unexpected discovery of of a large non-Terran source of phosphorus.
This is an independence war where the owning state is a megacorporation instead of a nation. This is uncommon in the real world but popular in science fiction. A corporation sets up something like an asteroid mining operation, develops it until it is near self-sufficient, and simultaneously downtrods the workers with all the savage explotation of a big business who knows it has the grabbed all the workers by the short-and-curlies. When the revolution happens, it is not so much a war as it is an incredibly violent labor strike.
Some revolutionary war features still obtain in a corporate war, such as the corporation controlling the rebel's access to elemental bottlenecks and the like. Some features do not, such as the fact since this is not a war between two nations, niceties such as the laws of war technically do not apply.
There will have to be a lot of improvising on the part of the rebels. Basically all they have in the way of military hardware is what they can cobble together out of equipment available at the mining operation, or whatever. This is why the rebels in Battlefleet Mars originally used quote "mining lasers" unquote to mine asteroids. The author figured adding a mining laser to a cargo ship would create instant warship.
A civil war is a war between forces belonging to the same nation or political entity.
When one contemplates the many long and bloody wars in Terra's history over differences in culture or the color of one's skin, one can only imagine the scale of wars possible with aliens from outer space. After all, warfare with enemy nation X is one thing, but at least they are human. Those things from Tau Ceti five are
freaking bug-eyed monsters extraterrestrial sophonts with unknown habits. It would be all too easy for combat to escalate to War to the Knife: total genocide.
There are some related motives for alien genocide, suggested as possible solutions to the Fermi Pardox:
and the latter few can involve aliens invading human settled planets, if not Terra itself.
Remember realpolitik. Ken does have a good point. The motivation of the invaders puts limits on the allowed invasion techniques. If the invaders want slaves, it is counterproductive to kill every living thing on the defending planet. If the invaders want real estate, it is counterproductive to dust the planet with enough radioactive material to render it uninhabitable for the next ten thousand years. And so on.
Not that aliens are likely to want slaves in the first place. Here and now we are already having problems with technological unemployment. Aliens technologically advanced enough to have starships will have zero need for human slaves when machinery can do any jobs faster, cheaper, and with no food required.
Aliens certainly don't want our women. That pathetic trope was tired and worn out back in 1958 when it was used in I Married a Monster From Outer Space. Let's get real here. An alien would find human women to be about a sexually attractive as humans would find a Komodo monitor. Yes there may be one or two with a kinky fetish, but certainy not species-wide.
It is highly unlikely that aliens would find humans to be a delicious food item, but even so it would be so much less trouble to swipe a few human cells and use them to grow vat meat.
If one must have aliens invading because they want some crucial resource, I like to use an analogy. Ordinary resources are not worth it. I don't care what you saw in the TV show V, they ain't gonna want Terra's water. Markus Baur points out that aliens invading Terra to steal our water makes about as much sense as Eskimos invading Central America to steal their ice. The rings of Saturn are almost pure water ice with a mass of fully half of Terra's entire Antarctic ice shelf, and it is already in space floating around free for the taking. The Jovian moon Europa has about three times the water as all the oceans of Terra combined.
The same goes for gold, uranium, or our women. But what if we hand-wave an unknown resource, something that our scientists have not even discovered yet? (Wow, Zzazel! Their planet is incredibly rich in polka-dotted quarks!)
Then us poor humans will find ourselves in the same spot as a primitive African tribe who does not understand why these Western stranger want to bulldoze their village in order to dig up the dirt. The westerners tell the tribesmen that the dirt is called "Coltain", from which they can extract something called "Tantalum", which is absolutely vital for something called a "Cell Phone." But to the tribesmen, it looks just like the same dirt that is everywhere else, and more specifically, in places that are not under their beloved village. This causes hard feelings, but unfortunately the westerners have something else called "automatic rifles".
Aliens could invade Terra by using exotic attacks, which at the time Terra does not realize they indeed are attacks.
We may very well have already been invaded by "aliens"
Footfall is arguably the best "alien invasion" novel ever written.