When it comes to conflict, it is a universal law. All things fight each other. Ranging from two bacteria, vying for the same food, all the way to unseen forces, fighting the entropy of the universe, or even fundamental concepts that will always conflict with each other, like creation and destruction. To wish for a peaceful, cooperative, symbiotic universe is ideal, yes. But reality states conflict will always be inevitable.
Putting grand concepts aside, locally conflict can always be observed, or even inferred. Volcanoes will spew forth lava, creating new land, which will inevitably be weathered, then eroded, by water, wind, and more. Creation will lead to destruction, but out of destruction can come more creation. Rocks can be eroded into soil, which will further nourish plants. Said plants can be killed, but out of that can animals be fed, trees turned into lumber, and more. And the animals themselves can become food to other animals. This is why evolution has dictated the need for weapons in that conflict, between predators and prey, regardless if the prey is animal or vegetable.
Both sides evolved stealth to hide from each other. Both sides have evolved speed to run away, or towards, each other. Both sides have evolved enhanced senses, to better find each other. But there are specifics as well. Prey evolved wide fields of vision, needing to be alert for predators at all times, hence eyes on the sides of their heads. Predators meanwhile needed the ability to judge distance, hence binocular vision and both eyes in front. Predators also evolved strong jaws, claws, teeth, as well as venom. Prey evolved detachable parts, as well as poison, to deter said predators. This has proven true on all observed worlds, even fictional ones. Conflict even occurs within an organism as well.
For the microbes always seek to have their way within a complex body, and seek to use its resources to their own ends. Therefore, the animals had to evolve immune systems to fend them back. Everything from the skin to the white blood cell is all there to protect against the conflict the microbes will bring, with many overlapping and helping each other when one fails. But all of the above conflicts are not what most are concerned about.
No, those who seek peace, cooperation, and symbiosis care mainly about one conflict, war. Now, some will also seek to end the need for conflict against animals, for food or other products. This is an admirable goal, yet it must be acknowledged that intelligence came from the flesh of animals. Intelligence has always come from the consumption of animal flesh. Pack intelligence is real, observed most famously in wolves and orcas. Packs can work together to bring down prey that one single animal cannot. Early apes and hominids did only eat plant material, but eventually discovered the advantages of consuming animal flesh, and the enhanced intelligence gained by consuming said flesh. The (likely accidental) invention of cooking forever anchored meat into the human diet.
But back to war. Early wars of man mirror those of the animals. They were often mostly over territory, or the resources of the land. Conflicts which can still be inevitable, as societies develop and grow, and people agree it is better to debate about such things around a table than on the battlefield. Later wars were about things that only humans care about, and do not share with animals. Primarily government, but also religion. Even when those wars fall out of favor, differences in government have always caused tensions, even if no actual fighting takes place.
Now the weapons used in war have always varied. From rocks and sticks, to swords and bows, to muzzle loading guns, to assault rifles, the weapons of war always change. But one constant is suffering. Death, disease, and uncaring leaders urging soldiers forward are all reasons why war is undesirable. It is why civilizations will crave peace. Even aside from the ethics, is the cost. The machines and weapons of war are always expensive to produce, and so too is the recruitment and training of soldiers. So it is always understandable why people seek to avoid war. And with a peace-loving society, news of their leaders sending them to war is never popular.
Even in peace, many will maintain strong militaries. This is a double-edged sword however. A military for peacekeeping is always a worthy goal. But it can be difficult for a people to accept a strong military with no enemy to fight. After all, a strong military, with no enemy to fight, in the wrong hands can lead to the people the military is supposed to defend, becoming the enemy of the state, or rather, the enemy of the despot seeking to maintain power. Anti-military cries can be strengthened when the society suffers other ills like poverty, homelessness, lack of education, and crime.
This is all understandable, and priority should be given to those when there is no enemy to fight. However, the intelligence network must never fail. Primarily to protect against small groups that seek to harm the people, such as terrorists and rebels. Care must be taken however, because it is not just important to know when and where such groups form, but why they do, and what their goals are. Care must be taken to ensure that the state is not the TRUE source of the conflict, and not just having to defend themselves against those that would mean them harm.
But even aside from all of that, even in a globally, or extra-globally, unified society, one that's ended all wars and conflicts within themselves, and has no further need of it, must always keep an active intelligence network, as well as the ability to train soldiers, and manufacture the machines of war, whenever needed. Because conflict can always come from the outside. True, other societies may be reasoned with. Sometimes it's after an initial conflict of interest. But such societies can always be reasoned with, eventually. Diplomacy can always find a way with said societies.
Yet even then, there are always two types that a society must always be weary of, and must always acknowledge are out there, waiting to conflict. Two types of Great Enemies that must always be pushed back, where no diplomat may win. Those two can be classified as world eaters, and world burners.
World eaters are enemies with desires that mirror those of animals and early man, yet never grow out of it. They will wander a world, stars, galaxies and even entire realms, looking for resources to consume. Those resources may be the planets themselves, or even specific resources found within. They may be after the people, seeking slaves, or even their matter. They may even desire metaphysical resources such as souls. Even those seeking to expand territory can be counted among the world eaters. World eaters may not always be societies either, at least not in the traditional sense. Examples of world eaters are everywhere in popular fiction. Halo's Flood count as world eaters, craving the biomass of all they encounter. So too the tyranids from Warhammer 40K, for the same reason. The Independence Day aliens are also world eaters. Star Trek's Borg are world eaters as well, among countless other examples. World eaters are likely the most common of the Great Enemies, because the audience can at least understand their goals. But they are not the most dangerous of the two.
That honor goes to world burners, so named after Alfred Pennyworth's line in the movie The Dark Knight. "Some men just want to watch the world burn." World burners seek to help entropy along, seek to undo creation, either for no reason other than to seek destruction for its own ends, or to destroy for goals that are unknowable to a society. World burners can also be world eaters, though often times the resource being consumed is unknown to the society the world burner is in conflict with. A good example of a world burner would be Mass Effect reapers, since to the galaxy at large, they crave no resources, only destruction. (And the whole "it's better if we do it, or your robots will do it," plotline IS something that to most would fall under the unknowable category, especially if smart AI is never developed. (Which is also a stupid plot. What the hell Bioware?)) The demons of Hell also primarily fall under this category, with some minor world eating happening as well in the form of souls. World burners can never be negotiated with, and are the primary reason a society should never forget how to wage a war.
The two Great Enemies are also the only reason why a society should develop Great Weapons. Smart weapons are always needed, and defeating armor and shields is always a requirement. However, there will always be weapons with destruction that is too great for most wars. Nuclear weapons are the classic example, as destroying a city will harm all civilians and non-combatants within that have nothing to do with the war, and is too much a travesty for use, and may only embolden resistance. But if a Great Enemy desires war, no weapon is too powerful. No weapon of mass destruction can be enough for a Great Enemy that is too powerful, or too destructive, to leave alone otherwise. Only in such cases, are nukes acceptable. Or beyond. Even planet killers like the Death Star can be wielded against a Great Enemy. Care must still be taken however, to prevent their misuse. Great Weapons must never be used against an enemy that is not a Great Enemy.
So to conclude, peace is desirable by all societies that seek to rise above animal instincts and urges, to fight back against the entropy of the universe. However, there will be a fight, and forces that always seek war. The most intelligent and peaceful can be brought down, simply by lack of readiness to fight the Great Enemies. The will to fight must always be present, the lessons of war must never be forgotten. Because those who do not prepare for inevitable war, whatever the war, or preparation, takes the form of, will be destroyed by those who embody conflict and destruction.