Caused by one country's wish to take control of another country's wealth. Whatever the other reasons for a war may be, there is almost always an economic motive underlying most conflicts, even if the stated aim of the war is presented to the public as something more noble.

Historical Examples of Wars Fought for Economic Gain
Anglo-Indian Wars (1766-1849) – The Anglo-Indian wars were a series of wars fought between the British East India Company and different Indian states. These wars led to the establishment of British colonial rule in India, which gave Britain unrestricted access to exotic and valuable resources native to the Indian continent.
Finnish-Soviet War or "The Winter War" (1939-1940) – Stalin and his Soviet Army wanted to mine Nickel and Finland, but when the Finnish refused, the Soviet Union waged war on the country.
    2.Territorial Gain
A country might decide that it needs more land, either for living space, agricultural use, or other purposes. Territory can also be used as “buffer zones” between two hostile enemies.

Related to buffer zones are proxy wars. These are conflicts that are fought indirectly between opposing powers in a third country. Each power supports the side which best suits their logistical, military, and economic interests.

Proxy wars were particularly common during the Cold War.

Historical Examples of Wars Fought for Territorial Gain
Mexican-American War (1846-1848) – This war was fought following the annexation of Texas, with Mexico still claiming the land as their own. The U.S. outfought the Mexicans, retaining Texas and incorporating it as a state.
Serbo-Bulgarian War (1885-1886) – Bulgaria and Serbia fought over a small border town after the river creating the border between the countries moved.
Arab-Israeli War or "Six Day War" (1967-1988) – Israeli forces took the territories of the West Bank, including East Jerusalem, from Jordan.
     3.Religion
Religious conflicts often have very deep roots. They can lie dormant for decades, only to re-emerge in a flash at a later date.

Religious wars can often be tied to other reasons for conflict, such as nationalism or revenge for a perceived historical slight in the past.

While different religions fighting against each other can be a cause of war, different sects within a religion (for example, Protestant and Catholic, or Sunni and Shiite) battling against one another can also instigate war.

Historical Examples of Wars Fought for Religion
The Crusades (1095-1291) – The Crusades were a series wars sanctioned by the Latin Church during the medieval age. The aim of crusaders was to expel Islam and spread Christianity.
Thirty Years' War (1618-1648) – When Holy Roman Emperor Ferdinand II tried to impose Roman Catholicism on the people of his domains, a faction of Protestants from the north banded together, sparking war.
Lebanese Civil War (1975-1990) – The Lebanese Civil War was primarily sparked from conflicts between the Sunni Muslim, Shiite Muslim and Christian Lebanese populations.
Yugoslav Wars (1991-1995) – The Yugoslav wars consisted of the Croatian War and the Bosnian War. The wars were fought between the orthodox Catholic and Muslim populations of former Yugoslavia.
Second Sudanese Civil War (1983-2005) – This ethno religious war was caused by the Muslim central government's choice to impose sharia law on non-muslim southerners.
    4.Nationalism
Nationalism in this context essentially means attempting to prove that your country is superior to another by violent subjugation. This often takes the form of an invasion.

Dr. Richard Ned Lebow, Professor of International Political Theory at the Department of War Studies, Kings College London, contends that while other causes of war may be present, nationalism, or spirit, is nearly always a factor. In his essay "Most wars are not fought for reasons of security or material interests, but instead reflect a nation's spirit," he writes:

Related to nationalism is imperialism, which is built on the idea that conquering other countries is glorious and brings honor and esteem to the conqueror.

Racism can also be linked to nationalism, as can be seen in Hitler’s Germany. Adolf Hitler went to war with Russia partly because the Russians (and eastern Europeans in general) were seen as Slavs, or a group of people who the Nazis believed to be an inferior race.

Historical Examples of Wars Fought for Nationalism
Chichimeca War (1550-1590) – The Chichimeca war was one of many wars fought during the Spanish conquest of the Aztec civilization in modern day Mexico.
World War I (1914-1918) – Extreme loyalty and patriotism caused many countries to become involved in the first world war. Many pre-war Europeans believed in the cultural, economic and military supremacy of their nation. 
     5.Revenge
Seeking to punish, redress a grievance, or simply strike back for a perceived slight can often be a factor in the waging of war. Revenge also relates to nationalism, as the people of a country which has been wronged are motivated to fight back by pride and spirit.

Unfortunately, this can lead to an endless chain of retaliatory wars being set in motion which is very difficult to stop.

Historically, revenge has been a factor in many European wars,

Historical Examples of Wars Fought for Revenge
World War II (1939-1945) – The rise of the Nazi Socialist Party and Germany's eventual domination of the European continent were direct results of the Treaty of Versailles, which imposed strict punishments on Germany.
War on Terror – The September 11th attacks on the World Trade Center in 2001 prompted President George W. Bush to initiate a war on terror. This global war began with an invasion of Iraq and is ongoing.
     6.Civil War
These generally take place when there is sharp internal disagreement within a country. The disagreement can be about who rules, how the country should be run or the people's rights. These internal rifts often turn into chasms that result in violent conflict between two or more opposing groups.

Civil wars can also be sparked by separatist groups who want to form their own, independent country, or, as in the case of the American Civil War, states wanting to secede from a larger union.

Historical Examples of Civil Wars
American Civil War (1861-1865) - The American Civil War was fought by the Union army and the Confederate army as a result of the long-standing controversy over slavery.
Russian Civil War (1917-1923) – The Russian Civil War followed immediately after the Russian Revolution, with the Red Army and the White Army vying to determine Russia's political future.
Spanish Civil War (1936-1939) – The Spanish Civil War was fought between the Republicans, who were loyal to the left-leaning Second Spanish Republic, and the Nationalists, a largely aristocratic conservative group led by General Francisco Franco.
Korean War (1950-1953) – The Korean War was a war fought between North Korea, which was supported by China, and South Korea, which was supported primarily by the United States.
     7.Revolutionary War
These occur when a large section of the population of a country revolts against the individual or group that rules the country because they are dissatisfied with their leadership.

Revolutions can begin for a variety of reasons, including economic hardship amongst certain sections of the population or perceived injustices committed by the ruling group. Other factors can contribute too, such as unpopular wars with other countries.

Revolutionary wars can easily descend into civil wars.

Historical Examples of Revolutionary Wars
Portuguese Restoration War (1640-1668) – The Portuguese revolution ended the 60-year rule of Portugal by Spain.
American Revolution (1775-1783) – The American Revolution gave the 13 North American colonies independence from British rule and established the United States of America.
French Revolution (1789-1799) – The French Revolution was a battle that represented the rise of the bourgeoisie and the downfall of the aristocracy in France.
Haitian Revolution (1791-1804) – The Haitian Revolution was a successful slave rebellion that established Haiti as the first free black republic.
     8.Defensive War
In the modern world, where military aggression is more widely questioned, countries will often argue that they are fighting in a purely defensive capacity against an aggressor, or potential aggressor, and that their war is therefore a “just” war.

These defensive wars can be especially controversial when they are launched preemptively, the argument essentially being that: “We are attacking them before they inevitably attack us.”