Has there ever been a truly multi-sided war?

Maybe the Great Northern War?

http://en.wikipedia.org/wiki/Great_Northern_War

Groups of Scandinavians went on viking raids in various christian kingdoms in western Europe from about 800 to about 1000, striking the British Islands, Germany, France, Spain and sometimes even the Mediterranean. It may be a matter of dispute how much those raids were war and how much they were crime. But certainly the times that viking groups tried to take over lands count as war.

Muslim pirates from various bases in the Mediterranean raided Mediterranean shipping and seacoasts. Muslim groups often took over various places in western Europe and used them as bases for further raiding. For example Fraxinet in Provence from 889-973. The Muslims of Fraxinet closed the passes of the Alps for decades.

In 842 a fleet from Muslim ruled Miseno near Naples landed at Ostia and sacked Saint Peter's in Rome.

The Magyars settled in Hungary about 900 and started raiding western Europe, not just Germany but also Italy, France, and even Spain.

Thus many regions of western Europe were subject to being raided by two or three different sets of external enemies, and that sometimes happened during civil wars or wars between kingdoms.

Thus there were possibilities for multi sided conflicts in that era.

For example, Wikipedia says:

There was a skirmish between Muslim and Hungarian cavalry in May 942, ended with a Hungarian victory.

https://en.wikipedia.org/wiki/Fraxinet[1]

And it seems very probable that Magyar/Muslim skirmish happened very close in time and space to Muslim/Christian conflicts and Magyar/Christian conflicts.

Wikipedia also says about King Conrad I the Peaceful of Burgundy (reigned 937-993) that:

According the chronicler Ekkehard IV, in a story that is probably apocryphal, when Conrad learned that both the Magyars and the Saracens of Fraxinetum were marching against him, he sent envoys to both armies warning them of the other. The envoys offered Burgundian aid to each invader against the other and then informed them of the other's whereabouts. When the Magyars and Saracens met, the Burgundians held back and only attacked when the opposing forces were spent. In this way, both invading armies were destroyed and the captives sold into slavery.[1][2]

https://en.wikipedia.org/wiki/Conrad_I_of_Burgundy[1]

This could be another version of the same story.

In any case, medieval writers could imagine three way conflicts happening during this era.

The war of the League of Cambrai may well suit since most factions within the war were at one time allies and at others enemies. Although there were two "clear" sides to the war.

The Eighty-Years' War / Thirty-Years War which ends in 1648 with the Westphalia Treaties was a very long conflict in which all European countries and peoples took part in various coalitions, alliances that changed over time.

It is widely considered as a religious War because the catholic European powers (Holy Roman Empire, Spain, etc) fought against Protestant enemies protected by Gustav Adolph of Sweden, later joined by Catholic France, who wants to counter-balance the power of the Austrian Habsbourg dynasty and prevent them from reaching hegemony in Europe through the transformation of the Holy Roman Empire in a "real" State (it was rather a confederation).

But it is also a struggle between the Holy Roman Empire and its northern and eastern dependencies and a political struggle in the Holy Roman Empire to strengthen the weak power of the Habsbourg Emperor.

However it is a major conflict in European history in which the alliances changed a lot. It led to the formation of a power structure in continental Europe that lasted until the Napoleonic Wars.

In the above answers I haven't seen the warring states period of China. It was like the real life version of the game Risk with seven (major) players for over 2 centuries till one of them conquered all the others at the end.

The biggest multiple side war was probably World War II. Biggest in all respects, including the number of sides.

Soviet Union started the war on the side of Germany; they jointly invaded Poland, and Soviet Union helped Germany against the allies.

Then Soviet Union started its own war with Finland. As a result, Finland became Germany's ally, but their participation in the war with Soviet Union was very limited. The Finns took back their territory and refused in further help with the invasion of Soviet Union.

In 1944 they switched sides and fought the Germans for a short time.

France first declared a war on Germany, then was defeated and become German ally. There were actually two governments of France, fighting on different sides. When Germany invaded Soviet Union, Ukrainians immediately tried to declare their independence and organized an independent army. At various times this army fought Germans and Soviets, but there was also a large scale Ukrainian-Polish war.

Similar story was with other nations whose main goal was fighting for independence.

Some nations of Soviet Union had approximately equal numbers of soldiers fighting of the German and Soviet sides. There was also a Russian army fighting on German side.

EDIT. Some commenters argue that German and Soviet co-ordinated invasion of Poland can be considered as "one side" in the war. OK, suppose that SU was a third side. This is another argument for saying that this was a multi-side war, if it was 3-sided from the very beginning. But I hope even those commenters will not try to argue that Soviet Union was a part on the US-British side in 1939-1940 (or that SU was not involved in the war at that time).

I am very surprised that my answer is qualified here as "full of cold war propaganda". On my opinion, I only stated the well known facts. But unfortunately stating of facts can be also considered as "propaganda" by some people, I had a lot of experience with this.

EDIT2. In 1941-1948 there was a real 4-sided war on the territory that is now known as Western Ukraine. The sides were: Germans, Poles, Ukrainians, and Soviet Union. Each side had an army; and all participants fought all other sides. For the details, see Timothy Snider, Bloodlands.

Currently ongoing Syrian Civil War is the best example. There are 4 fronts and none of them are event CLOSE to getting allied Government-Which is dictatorial, supported by Shia Hezbollah Opposition-Has both Islamist and Secular fronts, including Free Syrian Army and is supported by Al-Nusra, an al-Qaeda operative ISIL-Do I need to say anything? Kurdistan-Are a separate ethnic group which wants independence

The Syrian conflict is a great example, in fact I can't think of a conflict with more "players" both direct and indirect.

Directly though there are at least 6 different factions including the government, and within these factions there are even more smaller groups that keep changing allegiances based on the situation. This BBC article is a great example of the chaos. Please do note, it is 2 years old and some groups have morphed into others, switched allegiances, etc. But it illustrates quite well at that time just how many different groups there were fighting.

http://www.bbc.com/news/world-middle-east-24403003

Then of course you have The US and NATO keeping an eye on the situation focused now on groups like Al-Nusra Front and Daash/ISIL/ISIS, with Iran backing Assad via Hezbollah for the most part. Russia is a traditional ally of Assad, but not contributing much to the government other than diplomatically.

This seems to be most prevalent in civil wars of a country. The most recent example I can think of if the Angolan Civil War where the 3 main factions - UNITA, MPLA and FNLA fought eachother in equal measure and supported by various great/superpowers around the world. Of course at the end it became a massive proxy war between South Africa and Cuba.

In addition to Anixx's answer you should also be aware that in WW2, a similar thing happened, but this time the country was divided on political lines (Communist vs. non-communist) and both were against the occupying Germans. See this book.

Syrian civil war Syrian government and allies vs FSA and allies vs Kurds and allies vs ISIS and allies vs Al -aeda and allies

The Tripartite Struggle for Kannauj , is a very good example. There were 3 sides, The Pala rulers from Bengal, the Gurjara Pratihara of Rajasthan and West, and The Rashtrakuta, from South Or Deccan. The war continued for more than 3 generations on each side. Finally, The Pratihara rulers emerged victorious. This was an important era in history of India. The very era, from around 750 AD to 900 AD is named as Tripartite Struggle

Feudal Japan is also an intriguing example of what you are looking for.

The Genpei War was a conflict that featured three major belligerents: The Minamoto (Yoritomo) clan, The Taira clan, and the Minamoto (Yoshinaka) clan, all battling for dominance of the imperial court. To a lesser extent, the Fujiwara Clan, who had long been part of the ruling elite, participated in the political intrigue during this period.

The war in Japan during The Sengoku Period in particular was extremely fierce and protracted, lasting from the 15th to the 17th century. It also featured numerous powerful factions. The authority of both the shogunate and the Imperial Court collapsed, and provincial Governors (shugo) and other local samurai leaders emerged as the daimyo, who would battle each other, religious factions (e.g. the Ikkō-ikki) and others for land and power for the next 150 years or so.

Over one hundred domains clashed and warred throughout the archipelago, as clans rose and fell, boundaries shifted, and some of the largest battles in all of global pre-modern history were fought.

The second Sino-Japanese War and the Chinese Civil War may be a good example. There was a civil war between the Kuomintang (KMT) and the Communist Party (CPC) factions, and in the middle of this Japan invaded. Both KMT and CPC resisted the Japanese, and at some point they even (nominally) cooperated. This cooperation was brief, however, and there were major clashes between these factions since 1941 (while they were also fighting the Japanese).

The situation came to a head in late 1940 and early 1941 when there were major clashes between the Communist and KMT forces. In December 1940, Chiang Kai-shek demanded that the CCP’s New Fourth Army evacuate Anhui and Jiangsu Provinces. Under intense pressure, the New Fourth Army commanders complied, but they were ambushed by Nationalist troops and soundly defeated in January 1941. This clash, which would be known as the New Fourth Army Incident, weakened the CCP position in Central China and effectively ended any substantive co-operation between the Nationalists and the Communists and both sides concentrated on jockeying for position in the inevitable Civil War. It also ended the Second united front formed earlier to fight the Japanese. (wikipedia)

You should consider the Bosnian war (in Yugoslavia) in the 1990s, there were basically three factions fighting each other: the Orthodox Christian Serbs (Serbia), the Catholic Croats (Croatia) and the Slavic Muslims (Bosnia). The war was three-sided from 1992 to 1994, and two-sided in 1995-1996.

You can see that even the table of belligerents in the Wikipedia lists three sides.

For a relatively brief period in China's Three Kingdoms era, the three states of Wu/Shu/Wei were actively fighting each other. For most of the rest of the time, Wu and Shu were allied in their resistance against the vastly stronger Wei.

• In 219 (AD), the ill-fated Battle of Fancheng took place. Shu, fresh from their acquisition of Hanzhong, invaded Wei, but this campaign soon reached a stalemate.
• In the same year, sensing an opportunity, Wu successfully invaded Jing province, then under the control of Shu.
• In 221, wishing revenge and to retake the key province, Shu attempted to retake Jing province. This campaign lasted a full year until Shu were defeated in the Battle of Xiaoting.
• In 223, Shu and Wu reaffirmed their alliance.

So for a period of approx. 4 years, between the invasion of Jing province by Wu and the reaffirmation of their alliance, the three states of Wu, Shu and Wei could be said to have been at war with each other.

In terms of scale (keep in mind the numbers are inexact and may be unreliable due to the age):

• Battle of Fancheng involved 100k troops for Wei and 70k for Shu
• Battle of Xiaoting involved 50k for Wu and Shu suffered 80k casualties
• For reference, census results from the previous and following dynasties show the total population of China to be somewhere between 16 - 50 million.