What was the cleanest war ever fought?

I'd nominate the Western Front of World War I.

The conflict largely involved non-asymmetrical war between two sides.

Criteria:

Minimal civilian casualties: civilians were killed from the sinking of boats in the Atlantic - possibly ones that were carrying munitions. Also, Entente forces blockaded Germany, affecting its ability to import food. Away from the Western front, Turkey committed genocide, but I'm focusing on the Western front.

The war started with a declaration of war: yes.

No WMDs: Germany used chemical weapons.

The War ended with a treaty: The Treaty of Versailles.

No propaganda: there was a lot of persuasion, but I'm not aware of large-scale, sustained efforts at deception.

Parties can agree on the cause: I think so. Hostile military alliances.

Considering how many people died in WWI, I'm of the opinion that "cleanness", while measureable, isn't a very useful metric.

The cleanest war was probably the Cold War... because it was never fought.

There were casualties - the Soviets shot down quite a few US spy planes, and that was kept very quiet. And there were proxy wars, such as Korea and Vietnam, but overall, the Cold War was marked by an absence of armed conflict, avoiding the huge casualties and destruction of cities that normally come with a war.

In the end the Cold War was an economic war. And the western nations won it the capitalist way - they outspent the Soviets. Kruschev once famously said that when the time came to hang the western leaders, they would sell him the rope.

Unfortunately for him, he couldn't afford to buy it.

I think that the "Cabinet Wars" of the 18th century might fit in here.

[A]lso known as "war between princes." Such wars involved small armies, noble officer corps, limited war goals, and frequently changing coalitions among the belligerents.

As an example:

Berlin was not plundered during the Seven Years' War of 1756-1762, despite having fallen into enemy hands not once but twice.

I think the answer to this question is the Parsley/Perejil island conflict between Spain and Morocco in 2002 https://en.wikipedia.org/wiki/Perejil_Island_crisis.

Morocco occupied with an invasion force of 12 men an uninhabited island that Spain considered theirs. A week later Spanish special forces landed on the island and took the Moroccans prison without firing a shot. The prisoners were released and returned to Morocco the same day.

No deaths, no wounded, not even a shot fired.

The Dutch - Scilly Islands War or The 305-Year War comes to mind.

• zero civilian casualties
• started with a declaration of war
• no illegal weapons were used
• Ended with a treaty signed on 17 April 1986, 305 years exactly after beginning of hostilities.
• No propaganda was used - a reason why this war lasted that long; everybody had forgotten it!

I will add to the list the Toledo War. Both Michigan and Ohio raised militias with the intent to defend their respective claim to the Toledo strip. Shots were fired, although these were later claimed to be just warning shots over the heads of those who were already retreating. Apparently there was exactly one casualty: a stab from a pocket knife, resulting in a non-life-threatening wound. The conflict ended with a concession where Ohio got Toledo and Michigan got its upper peninsula.

About the only one of your criteria this war does not meet was the lack of propaganda. Both sides passed "laws" forbidding residents of the strip from paying taxes to the other, spread rumors about the strength and abilities of its militia, and so on. Mostly it was political blustering.

I'd name the Sonderbund War in 1848 in Switzerland. The catholic Swiss cantons attacked the protestant Swiss cantons because they wanted to separate.

However, three weeks after it broke out, the cantons of Fribourg and Lucerne was successfully defeated by governmental forces and the other catholic cantons didn't want to continue to rebel. The war ended with a peace treaty that consolidated the country as a single country instead of a federation of independent states. The war did exactly 93 victims.

There was also the Falklands War in 1983 in Falklands islands, opposing UK and Argentina. Some soldiers died, even horribly; However the war was short, and on both sides there was not really hatred for the enemy but rather the obligation to obey to their respective government and conquer the island. The war was too short to escalate into something really messy like it usually does, with a spiral of torture, hatred, harm to civilians, etc...

This war meets pretty much all standards for "cleanness" given by the OP. There was 3 civilian victims and 904 military victims.

How about Flower Wars, between the Aztec and their enemies? These were conducted according to very strict conventions. There were limited combatants and the location was preselected. The aim was to gain sacrificial victims, and early in the wars casualties were low (they got higher as the war went on, though). Much of the violence was the sacrifice (later) of prisoners and not from the battle itself.

The Slovenian war of independence, which was fought in 1991 was pretty clean by your standards.

Civilian casualties are stated as 12 foreigners who strayed into the line of fire, and there were also a few Slovenian civilian casualties. But combined these were much less than the 63 military casualties. The Yugoslav army destroyed some civilian property, such as parts of an airport and some passenger planes on the ground, and they also targeted TV transmitters. After ten days of fighting, a cease-fire was declared and the Yugoslav army retreated to Croatia, where a much bigger war was brewing.

There was no official declaration of war, but by declaring independence, the Slovenian authorities knew what would follow and were prepared.

No illegal weapons were used.

The war ended with a treaty.

All propaganda was kept at normal levels.

the two sides agree that the cause of the war was Yugoslavia's unwillingness to let Slovenia declare independence.

The Anglo-Zanzibar War fits your criterion, in part because it was so short, but it was also conducted with civil restraint. The new Sultan was suspected of assassination and violated a British treaty by occupying the palace. The British attempted negotiations and finally issued an ultimatum to vacate the palace at 0900. Half an hour before a final attempt at negotiation was attempted and the British made it clear they would open fire.

The palace grounds were fired upon. A single Zanzibar naval vessel was sunk when it fired at the British fleet. Surrender was accepted 38 minutes later. The British landed troops to help put out the fire and patrol the streets. The sultan fled to the German consulate who escorted him out of the country.

About 500 people were killed in the bombardment and subsequent fire. It's difficult to know who was civilian and who was not; the palace was defended, in part, by a hastily raised militia. The British gave ample warning of their intent to fire for them to have been evacuated.

The First Barbary War between the United States and the Barbary pirates matches your criterion. The cause was clear: Tripoli demanded their traditional protection money from the US and the US refused to pay. Tripoli declared war on the US by chopping down the flag in front of the US consulate (according to Wikipedia, this is traditional). The US Congress did not formally declare war, but ordered that armed American vessels were to to seize all vessels and goods of the Pasha of Tripoli "and also to cause to be done all such other acts of precaution or hostility as the state of war will justify." Tripoli harbor was blockaded by a multinational force and raids were conducted against their fleet.

After years of blockades and raids, a US mercenary force approached Tripoli by land via Derne. The US commander requested safe passage and supplies. The city governor refused, reportedly with "My head or yours!". The American's target was a fort and the governor's palace. I have no mention of civilian casualties, but city battles are never pretty, and mercenaries aren't the best behaved.

Soon after the capture of Derne, with Tripoli threatened and the blockade being bad for business, the pasha surrendered. A treaty was signed declaring a "inviolable and universal peace, and a sincere friendship", exchanging all prisoners, ending the blockade, and withdrawing from Derne. Curiously, after refusing to pay tribute, the US agreed to pay ransom for American prisoners.

The Anglo-Swedish war of 1810-1812. A phoney war forced upon Sweden after the devastating defeat in the Finnish war; neither side wanted to fight the other, and no battles were fought. There were, however, a formal declaration of war and a signed peace, and British troops that were stationed at the Island of Hanö occupied it during the war.