Has any ideology ever had an internal war?

• The Sino Soviet conflict - two communist nations at war
• Hungarian Revolution - a communist nation assisting another communist nation to suppress a communist uprising.
• Wikipedia: Communist Nations at War

@LangLangC provides several other examples that I think are worth pulling out of comments and into answer. (LangLangC and I independently came up with the Hungarian revolution).

The reverse-Rosato?: GDR, Hungary in the 50s, Poland in the 80s, and especially CSSR 68: all kept quiet with armed forces. For CSSR it was minute ideological differences seen as a big threat leading to a real invasion. Trying to uphold the argument in Q is between again pattern recognition and biased sampling (your untrue Scot) to try to intellectually polish an unsustainable ideology itself.

Quoting from the Democratic Peace Theory Wikipedia page,

Regarding specific issues, Ray (1998) objects that explanations based on the Cold War should predict that the Communist bloc would be at peace within itself also, but exceptions include the Soviet Invasion of Afghanistan, the Cambodian-Vietnamese War, and the Sino-Vietnamese War. Ray also argues that the external threat did not prevent conflicts in the Western bloc when at least one of the involved states was a nondemocracy, such as the Turkish Invasion of Cyprus (against Greek Junta supported Cypriot Greeks), the Falklands War, and the Football War. Also, one study (Ravlo & Gleditsch 2000) notes that the explanation "goes increasingly stale as the post-Cold War world accumulates an increasing number of peaceful dyad-years between democracies". Rosato's argument about American dominance has also been criticized for not giving supporting statistical evidence (Slantchev, Alexandrova & Gartzke 2005). Democratic Peace Theory

The list of leaders of the Soviet Union reveals repeated internal conflicts. OP may argue that these do not constitute full scale wars, but the Trotsky required an assassination, and Gorbachev required troop movements. Yanayev coup also seems to be a good counterexample.

This also ignores civil wars and conflicts. The American Revolution was fought between two democratic states. (although Great Britain had a king, Great Britain was actually a parliamentary democracy). The American Civil War is another example - both sides were representative democracies. Nearly ever civil war is between two sides that share an ideology. (which is why I object to the term ideology - since you can always invoke the "No True Scotsman" to remove inconvenient counterexamples.

Question: Has any ideology ever had an internal war?

Democracies are alleged to never declare war on each other. Is this true for any ideology? No Stalinist states have ever fought each other? No maoist states have fought each other? No fascist states? Etc.

Exclude theocracies and monarchs who rule by divine right.

Short Answer:

I see this question consisting of Three parts.

1. Has any ideology ever had an internal war?

   • Yes Famously for both the Nazi's(Night of the Long Knives) and Communists(The Great Terror). But probable to numerous other examples to list here.

2. Democracies are alleged to never declare war on each other.

   • Yes allegedly, but No historically

That theory with regards to Democracies was proposed in a time when few democracies existed. It was proposed by enlightenment philosophers who were trying to sell democracy. They were listing all of the perceived benefits to their idealized form of government. The theory stated that Democracies would not war upon each other due to special properties of Democracies. Three years after the theory was proposed it was dealt a pretty significant blow in the Quasi War between France and the United States, two Democracies. Since that time history has demonstrated many examples of Democracies waging aggressive wars against each other.

As for other less representative forms of government, they have never thought to be less prone to hostilities against each other. I list a few be low.

Nazi's(Fascists)

• In 1934 the SA or Nazi Brown Shirts lead by Ernst Röhm consisted of 3 million men. They saw themselves as the "Peoples Army" and the replacement for the Reichswehr, German Army. The German army which the SA wanted to absorb was fewer than 100,000 men at the time. When Röhm's set himself up as a rival to Hitler, Hitler took action and murdered Röhm and some 200 others in the SA leadership. After that the SA was diminished in importance in favor of the more loyal and fanatical SS.

Communists

• The Great Terror 1936 in Stalin's Russia. Stalin moved to consolidate power. Approximately 1.6 million people were arrested, 700,000 were shot, and an unknown number died under torture many of them former Communists.
• Sino-Soviet border War (1969) seven month long war between the Communist Soviet Union and Communist China along their mutual boarder at the height of the Sino-Soviet split.
• Sino-Vietnamese War (1976) war between Communist China and Communist Vietnam.
• Cambodian Vietnamese War (1978) between Communist Vietnam and Communist Cambodia(Khmer Rouge).

More Detailed Answer:
What you are brushing up against is called the Democratic Peace Theory. First proposed by philosopher Immanuel Kant and political theorist Thomas Paine in the late 1700s. see Immanuel Kant essay "Perpetual Peace: A Philosophical Sketch" written in 1795.

The theory says the factors which inhibit Democracies(conflating Democracy and Republic for a moment) over other forms of governments from engaging in war:

• Democratic leaders are held responsible for war casualties at the ballot box.
• Democracies because they are accountable by nature invest more in diplomatic institutions, which aid in resolving issues before wars can occur.
• Democracies don't view other Democracies with similar policies as threatening.
• Democracies tend to be wealthier than nonDemocracies, and either as a cause or a result are motivated to preserve infrastructure and resources which wars consume.

One problem with Emanual Kant and Thomas Paine's premise is it's mostly theoretical as there weren't many Democracies in existence when they first speculated on this benefit of a more representative approach to governence. The United States Constitution was ratified June 1788. Frances Bastille Day was 14 July 1789. The only other Republic at the time was the Republic of Ragusa, centered around modern day Dubrovnik Croatia, it was basically a city state. Ragusa while being around for 450 years blinked out of existence in 1808 during the Napoleonic Wars. That the new United States in the late 1700's and the tiny Republic of Ragusa didn't go to war hardly proves Kant and Paine's premise that republics won't war against each other. Three years after Immanuel Kant's essay was published however, The United States and the Republic of France did have a conflict resulting in the Quasi War.

To take a more historical look at Democracies specifically ... From List of wars between democracies

• The Peloponnesian war among the Greek City States. Athens fought against many other democracies in that war.
• The First Second and Third Punic Wars between the Roman Republic and the Carthaginian Republic.

18th Century

• The Quasi War between the US and France previously mentioned. In 1798 The United States would lose 2000 merchant ships to the Republic of France in the Quasi War.

19th Century

• War of 1812
• Mexican–American War
• Sonderbund War
• War of 1849 between the Roman Republic and the French Second Republic
• American Civil War, the United States was a republic and the Confederate States was nominally a republic as well
• War of 1859 between Peru and Ecuador
• Spanish–American War
• First and Second Boer Wars

20th Century

• First Balkan War (1912–13)
• First World War
• Polish–Lithuanian War (1920)
• Continuation War
• First Kashmir War
• Six-Day War: The Lebanese air force intervened against Israel, both then being democratic states
• Yugoslav Wars: Bosnia, Croatia and Serbia were all multiparty democracies.
• Cenepa War(1995)

21st Century

• Russo-Georgian War(2008)

Wars between republics are fairly common.

• The Roman Republic engaged in three epic wars with the Punic Republic (Carthage): the First, Second, and Third Punic War.

• The First Anglo-Dutch War, 1652-1654 was fought between two republics: the Commonwealth of England, existing 1649-53 and again 1659-60, and the Dutch Republic.

• Beginning in 1938, The Weimar Republic of Germany attacked and subdued in succession:
  -- The First Czechoslovak Republic in March 1939;
  -- The Second Polish Republic in September, 1939;
  -- The French Third Republic in May 1940;
  -- The socialist Republic of the Soviet Union in June 1941.

  Yes, Nazi Germany was still officially the Weimar Republic. Dictatorships commonly have the official trappings of being a republic, except that the constitutional checks and balances have broken down. Likewise for socialist republics, except that only one political party exists.

• In June 1950 the Democratic People's Republic of Korea (North Korea) attacked without warning the Republic of Korea (South Korea), launching the Korean War. In quick succession the republics of the United States of America, the socialist Republic of the Soviet Union, and the People's Republic of China joined in, to varying degrees.

• In 2014 the Russian Federation attacked and subsequently occupied parts of the Republic of the Ukraine, a conflict that continues to simmer as of writing.

I could go on (and on and on) but hopefully the point is made.

When the statement is made that "Democracies are alleged to never declare war on each other", a conflation has occurred between governmental form and economic form. What is really meant, and may be supportable in general, is that nations with open market capitalist economies rarely declare war on each other. In all the modern examples above either both, or at least the aggressor of, the involved nations possessed protectionist, socialist or mercantalist economies that see themselves in competition for economic gain rather than in cooperation for the same.