Civilization based on the teachings of Buddha

You said "civilization", not "a country". You also said "based on" - not mere "practicing".

Of course, if the entire civilization were based on the authentic Dharma, there wouldn't be a need for military force because in a Dharma-based civilization conflicts would be prevented and resolved long before they become violent.

Understand, Buddhism is all about preventing conflict in its various forms because conflict is the essence of dukkha. Mahayana Buddhism teaches the arising of conflict: fundamentally it comes from attachment to simplified overgeneralized concepts that clash with each other and with reality. In a civilization based on the teaching of Buddha, the role of conceptual overgeneralization in the arising of conflict would be as clear to everyone as the role of germs in spreading disease is clear to our civilization today.

As the danger of not hearing each other due to attachment to concepts would be clearly understood, people would take care to listen to each other and understand each other's perspective. In a Buddhist civilization, appreciation of other perspectives would be the mainstream philosophy and this alone would prevent majority of conflicts and remove the need for the military force.

Not to mention other Buddhist teachings such as loving-kindness, compassion, minimalism (being happy with little), and boundlessness aka interbeing.

It's worth remembering that Chinese martial arts flourished in Buddhist monasteries, usually for the defense of monastic lands (which were comparatively valuable). Buddhists have rarely been overly aggressive, but they weren't completely passive in the face of aggression, either.

It's worth noting that the 'dangers' of fire, poison, and weapons aren't the physical dangers of such things. The danger is mental attachment: finding fire, poison, and weapons fascinating or beautiful; taking them as tokens of power and virility. A sword and a plow are both tools, but a sword (unlike a plow) can give a sense of power over others, and that can lead to all sorts of craving and delusions. That doesn't mean that swords should necessarily be shunned, though. It merely means we should be careful to treat swords the way we treat plows: as tools that we take out at need and then store away again.

Perhaps that line is misinterpreted.

Here is a translation of the sutta, AN 11.15 or AN 11.16, in full:

“Mendicants, you can expect eleven benefits when the heart’s release by love has been cultivated, developed, and practiced, made a vehicle and a basis, kept up, consolidated, and properly implemented.

What eleven? You sleep at ease. You wake happily. You don’t have bad dreams. Humans love you. Non-humans love you. Deities protect you. You can’t be harmed by fire, poison, or blade. Your mind quickly enters immersion. Your face is clear and bright. You don’t feel lost when you die. If you don’t penetrate any higher, you’ll be reborn in a Brahmā realm. You can expect eleven benefits when the heart’s release by love has been cultivated, developed, and practiced, made a vehicle and a basis, kept up, consolidated, and properly implemented.”

I suppose that is saying that the mind will not be harmed -- e.g. harmed by anger, harmed by fear, etc.

From the Dhammapada, Chapter 3 about the mind:

36. Let the discerning man guard the mind, so difficult to detect and extremely subtle, seizing whatever it desires. A guarded mind brings happiness.

37. Dwelling in the cave (of the heart), the mind, without form, wanders far and alone. Those who subdue this mind are liberated from the bonds of Mara.

38. Wisdom never becomes perfect in one whose mind is not steadfast, who knows not the Good Teaching and whose faith wavers.

39. There is no fear for an awakened one, whose mind is not sodden (by lust) nor afflicted (by hate), and who has gone beyond both merit and demerit.

40. Realizing that this body is as fragile as a clay pot, and fortifying this mind like a well-fortified city, fight out Mara with the sword of wisdom. Then, guarding the conquest, remain unattached.

41. Ere long, alas! this body will lie upon the earth, unheeded and lifeless, like a useless log.

42. Whatever harm an enemy may do to an enemy, or a hater to a hater, an ill-directed mind inflicts on oneself a greater harm.

I think it's the mind that's the focus of the early Buddhist teachings -- more-so than geopolitics:

1. Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with an impure mind a person speaks or acts suffering follows him like the wheel that follows the foot of the ox.

2. Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with a pure mind a person speaks or acts happiness follows him like his never-departing shadow.

3. "He abused me, he struck me, he overpowered me, he robbed me." Those who harbor such thoughts do not still their hatred.

4. "He abused me, he struck me, he overpowered me, he robbed me." Those who do not harbor such thoughts still their hatred.

5. Hatred is never appeased by hatred in this world. By non-hatred alone is hatred appeased. This is a law eternal.

6. There are those who do not realize that one day we all must die. But those who do realize this settle their quarrels.

There is a lot of doctrine about "society" e.g. the Vinaya. But consider the Punna Sutta, (SN 35.88):

Good, Punna, very good. Possessing such calm and self-control you are fit to dwell among the Sunaparantans. Now it is time to do as you see fit.

So I think that's how we're supposed to learn to live -- i.e. empty-handed, not carrying weapons around with the idea of practicing "self defence".

To answer your question though, I think that every country, empire, or civilisation does or did have a military even if it is based on the teachings of Buddha -- even Bhutan now, apparently, though I don't know why. Maybe it's a requirement for Sovereignty in modern international law.

Is it, "to protect the citizens from any aggression"? Canada for example has an army, which has fought: but never in North America, in the last 150 years or more. It's arguably "metta" (not that Canada is officially Buddhist) that "protects Canadian citizens from any aggression" from the United States for example.

A more common reason for armed force in society is I guess not military but police. I think (but I can't find the reference) that Buddhist monks are advised to avoid personal danger by avoiding places where are there are known to be dangerous bandits.

I supposed that "known to be dangerous bandits" is just the sort of event or place where a "king" will sent a "police" force -- to protect citizens.

I think that in a Buddhist context you could say there are two co-existing societies, i.e. monks, and lay-people. Monks are generally very much unarmed (perhaps a few historical anomalies e.g. the Japanese Sōhei) -- and protected by lay society (and perhaps also protected by "Deities" as quoted in the metta sutta above).

Then again the ideal monk might avoid being a target for bandits by having little they might want to steal. I suppose they could be a target for an evil bully's sheer cruelty, even in that case there's Buddhist doctrine called the Parable of the Saw:

"Monks, even if bandits were to savagely sever you, limb by limb, with a double-handled saw, even then, whoever of you harbors ill will at heart would not be upholding my Teaching. Monks, even in such a situation you should train yourselves thus: 'Neither shall our minds be affected by this, nor for this matter shall we give vent to evil words, but we shall remain full of concern and pity, with a mind of love, and we shall not give in to hatred. On the contrary, we shall live projecting thoughts of universal love to those very persons, making them as well as the whole world the object of our thoughts of universal love — thoughts that have grown great, exalted and measureless. We shall dwell radiating these thoughts which are void of hostility and ill will.' It is in this way, monks, that you should train yourselves.

"Monks, if you should keep this instruction on the Parable of the Saw constantly in mind, do you see any mode of speech, subtle or gross, that you could not endure?"

"No, Lord."

"Therefore, monks, you should keep this instruction on the Parable of the Saw constantly in mind. That will conduce to your well-being and happiness for long indeed."

That is what the Blessed One said. Delighted, those monks acclaimed the Teaching of the Blessed One.

Like I said I think that -- at least for monks -- it's primarily protecting the mind, not protecting the body or personal wealth. And it's protecting the mind from "experiencing" cruelty i.e. from actually being cruel itself.

I believe the answer to your question is contained in the Metta Sutta itself if you view it from a particular perspective.

Consider that resolving the inner conflict which we perceive as suffering, requires us to first open our mind to seeing things the way they actually are.

Consider that opening our mind to seeing things the way they actually are requires us to first open our heart so that we can release our craving and clinging to desired sensory experiences.

Consider that the Metta sutta, like all suttas, is meant to be memorized and recited and recitation is meant to guide us towards solving a particular problem.

Consider that the Metta sutta is structured to

first open the heart so that

craving for and clinging to the desired sensory experience can be released, so that

the mind can then be stilled to the point where the inner conflict (which we perceive as suffering) can be clearly discerned, so that

the inner conflict can be resolved by discarding the āsava (sabbe saṅkhāra annica),

and we might be reborn into a new world (perceiving sensory experience from a new perspective)

AKA “seeing the world the way it actually is”.

If you view it in this way, and recite the sutta, while the inner conflict which you perceive as suffering present, with the intent of allowing the Buddha’s words to guide you towards (1) clearly seeing the nature of the conflict and (2) resolving it, insights will naturally arise to enable you to do so.

Consider that a world where technology is advancing faster than the moral systems needed to guide their usage is likely to destroy itself. However, if the minds of the people were fully liberated, we might not only be able to stop the world from killing itself. We might also be able to put an end to old age, sickness and death.

Finally, consider the possibility that the world which the Buddha described in the Metta Sutta was the world which we should actively strive to actually bring into being.

What preconditions would need to exist in order for such a world to emerge?

Mettā Sutta

This is what should be done
By those who are skilled in goodness,
And who know the path of peace:
Let them be able and upright,
Straightforward and gentle in speech.
Humble and not conceited,
Contented and easily satisfied.

Unburdened with duties and frugal in their ways.
Peaceful and calm, and wise and skillful,
Not vain and demanding in nature.
Let them not do the slightest thing
That the wise would later reprove.
Wishing: in gladness and in safety,
May all beings be at ease.

Whatever living beings there may be;
Whether they are weak or strong, omitting none,
The great or the mighty, medium, short or small,
The seen and the unseen,
Those living near and far away,
Those born and to-be-born,
May all beings be at ease!

Let none deceive another,
Or despise any being in any state.
Let none through anger or ill-will
Wish harm upon another.
Even as a mother protects with her life
Her child, her only child,
So with a boundless heart
Should one cherish all living beings;

Radiating kindness over the entire world:
Spreading upward to the skies,
And downward to the depths;
Outward and unbounded,
Freed from hatred and ill-will.

Whether standing or walking, seated or lying down,
Free from drowsiness,
One should sustain this recollection.
This is said to be the sublime abiding.

By not holding to fixed views,
The pure-hearted one, having clarity of vision,
Being freed from all sense desires,
Is not born again into this world.

Would a civilization based on the teachings of Buddha ever had a need for military force?

People who practice metta are always a minority in this world. And the people who attain a successful level of metta are minority of minority.

If we consider about the ancient kings:

1. The King Bimbisara who had unwavering faith and was the main upasaka-king, had a military.
2. The King Asoka who was the Main emperor in Buddhism, had a military.

It is said in Metta Sutta that one who practices metta is free from dangers of fire, poison and weapons.

Such a person is secure while in the Metta Jhana, I think.

Otherwise how can the "top disciple venerable Moggallana" was attacked to death by a gang of robbers.

Even the "top Upasika in metta" was burnt alive by another wife of her husband king.

Kamma can not be overpowered always by metta, I think.

Is the practice of metta enough to protect the citizens from any aggression?

If one practice it properly, he will get huge protections from the dangers than a non-practitioner.

But everything can not be done by metta alone, I think.