Like it or not, war shapes the world. Wars and conflicts decide political boundaries, move populations around the world, define political debates, and generally affect just about everything in our societies. Understanding world history would be impossible without understanding the conflicts that have shaped it.

If you’re interested in a crash course on world history, you may want to check out Brainscape's complete set of History flashcards. You'll definitely be able to show off your history knowledge in front of your friends after this.

In the meantime, here's a list of 13 conflicts and wars in history that have shaped our modern world to use as a starting point.

1. The Crusades

During the Middle Ages, the Roman Catholic Church declared a series of holy wars, or Crusades, which drew volunteers from the common people and military bodies of all the European nations. For some 200 years between 1095 and 1291, multiple crusader armies would march southeast to Asia Minor and the Levant, where they would do battle with Islamic forces in an attempt to gain control of the Holy Land.

The Crusades were ultimately unsuccessful, and it is estimated that some 3 million people were killed in the wars. Other minor crusades were also declared during this period against non-Christian peoples in several regions.

2. The French Revolution

Beginning in 1789, French society was wrought by social and political turmoil that would last for a decade and change human history. Perhaps the most central legacy of the Revolution is the move away from theocracies and monarchies. The French Revolution in many ways gave birth to the modern democratic republic.

The Revolution is thought to have been triggered at least partially by regressive taxation of the common people in an attempt to deal with French debt. The French Revolution is widely considered to be one of the most important political events in human history.

3. The Mexican-American War

In the early 1800s, the territory of Texas was claimed by three nations: the Spanish, the French, and the Americans. Over the next 30 years, Mexico gained control, but thousands of American immigrants settled in the region.

These immigrants eventually rebelled against the Mexican state in 1835, winning independence as the Republic of Texas. The Republic of Texas existed until 1846 when it officially became part of the United States. Border disputes with Mexico escalated and led to war in 1846.

Over the next two years, the American forces defeated the Mexicans and forced the cession of what is now New Mexico, Arizona, southern California, parts of Texas, and other areas of the southwest United States.

4. World War I

The image we all have of World War I is that of trench warfare: ugly, dirty, brutal stalemates. Indeed, this did characterize much of the war. But the history of the conflict is much deeper than this. Even understanding the roots of the war requires a great deal of study, as it involves the culmination of decades of treaties and tensions between the nations of Europe and western Asia.

It is estimated that somewhere around thirty million people were killed in the Great War, which was so horrific many believed it would be the last war ever fought.

5. World War II

One of the results of World War I was the formation of the League of Nations and the imposition of severe economic sanctions and debt-repayment obligations on Germany and the other Central Powers. These fed a festering resentment in Germany after World War I that would lead to WWII.

After Hitler came to power and invaded Poland in 1939, the battle lines were soon drawn. Imperial Japan was aggressively expanding in the East, while Italy under Mussolini joined Hitler. Before the war was over, some 70 million people would lie dead and the ramifications of the war would spread widely.

In Nazi Germany, Hitler and his followers twisted German folk stories and traditions to create a vicious narrative that blamed Jews, homosexuals, blacks, communists, the mentally ill, and other “undesirables” for all the ills affecting Western civilization. Under the direction of propaganda minister Joseph Goebbels, the state constructed the ideological basis for disregarding these peoples.

The state then proceeded to seize their wealth and property, send many of them to forced labor camps to support the German war effort, and to directly exterminate them at concentration camps. 11 million people were killed in the Holocaust, including 6 million Jews.

While World War II was still in progress, President Franklin D. Roosevelt began to bring together foreign heads-of-state to discuss the creation of a successor to the League of Nations. What emerged from those talks was the United Nations, which was established in October 1945. The UN now has 193 member nations and serves as an important arbiter of international relations, taking on issues related to war and conflict, disaster relief, environmental concerns, and human rights.

6. Israel and Palestine

Since the 1917 Balfour Declaration, Jews had been moving to Palestine, which at the time was controlled by the British. By 1948, hundreds of thousands of Jews lived there. The UN resolved to partition Palestine, the British pulled out, and the State of Israel was declared.

From the beginning, it was a bloody affair. The surrounding Arab states attacked Israel and the fighting dragged on for a year. Since then, Israel has lived in a state of more-or-less constant conflict with its neighbors and with the Palestinian peoples.

7. The Cold War

rockets from the cold war

After World War II, the European nations which had been in power were eclipsed by two nations: the USSR and the United States. Both were strategically poised to take advantage of the war’s aftermath for economic and political gain, but both were wary of one another since the USSR espoused communism, while the United States championed capitalism.

In the wake of the war, neither country was itching for a fight, and the conflict settled into an uneasy “Cold War,” in which no overt battles were fought. However, the Cold War (which lasted from 1947 to 1991) was characterized by numerous “proxy wars” such as Korea, Afghanistan, and Vietnam, political wrangling, and an arms race which included the development of massive nuclear weapons arsenals.

The Soviet Union eventually collapsed in 1991, exhausted politically and financially from the decades of conflict with the West.

8. The Korean War

Japan ruled Korea until the end of World War II, which led to upheaval. The country was divided between North and South Korea, with the north occupied by the USSR and the south by the United Nations (led by the United States). By 1948, tensions rose, as both proxy governments wanted to reunite the country.

In 1950, war broke out as the North invaded the South with support from China and Russia. The war ended by 1953 when a new stalemate had been reached, thanks to the heavy involvement of U.S. troops. The demilitarized zone between North and South Korea is the legacy of this war.

9. The Vietnam War

The roots of the Vietnam war lie in the French colonization of Indochina, now Vietnam, Laos, and Cambodia, in the late 1800s. Over the next fifty years, resentment festered among the Vietnamese especially, leading to the creation of the Viet Minh, who rebelled against French control. That war lasted seven years, during which time the nationalist Vietnamese made an alliance with the Communist powers of China.

This drew in the Americans, who saw the war as another front in the war against the spread of communism and a key element in the “containment” approach. Twenty years and more than 350,000 U.S. casualties later, the U.S. pulled out.

10. The Soviet-Afghan War

In the late 1970s, Afghanistan was a country in some turmoil. After being ruled by a king for 40 years, two coups occurred in quick succession, putting a new socialist government in power. New policies by the government, which had close ties to the USSR, were unpopular with the people, and led to rebellion, civil war, and ruthless crackdowns.

In June 1979, the first Soviet troops entered the nation. They were quickly drawn into a guerrilla war of attrition with Russians and government forces controlling the cities. Insurgents (supported by funding and weapons from the West) controlled the outlying regions. The Russians pulled out in 1989.

This war had momentous consequences; it contributed to the collapse of the Soviet Union and led to the creation of al-Qaeda under Osama bin Laden (who was a member of the Afghani resistance), setting the scene for the later U.S.-Afghan War.

11. The Iraq-Iran War and the Gulf War

Iraq has been the site of many wars in the past decades. Prompted by the Iranian Revolution and a desire to gain power in the region, Iraq invaded Iran in 1980. The following war lasted 8 years and ended in an inconclusive ceasefire.

The United States supported Iraq during the war, but it was barely a few years later when the tides would turn. In 1990, Iraq invaded Kuwait, which held $14 billion in Iraqi debt from the last war. The small Kuwaiti armed forces were quickly overwhelmed. But in January 1991, a coalition of nations led by the United States led a 6-week offensive that decisively defeated the Iraqi military and forced them back.

12. The U.S.-Afghan War

After the September 11th terrorist attacks, the United States demanded that Afghanistan turn over Osama bin Laden. The Taliban, a conservative Islamic organization which had come to power in 1996, refused to turn him over, and the the U.S.—supported by the United Nations—invaded.

The war that followed would be the longest in U.S. history. As of today, most U.S. and coalition troops have finally been withdrawn from Afghanistan. Osama bin Laden is dead, but the security and stability of the nation is poor.

13. The Iraq War

In 2003, under the rationale that Saddam Hussein’s Iraqi government possessed weapons of mass destruction and was poised to attack the United States or other allies, a U.S.-led coalition launched an invasion of Iraq. The invasion quickly overwhelmed the Iraqi military, and Saddam was captured (and was executed in Iraq after a trial in 2006).

Despite the initial military victory, U.S. involvement was marked by extensive insurgency, which resulted in the deaths of more than 6,000 coalition forces. Post-Saddam, the violence has also resulted in the deaths of more than 17,000 Iraqi government forces and more than 110,000 civilians.

Due to the lack of evidence that Iraq posed a threat to the U.S. or other nations, the instability that still plagues the nation, and an estimated cost of nearly $2 trillion, many compare the Iraq War to a modern-day Vietnam War.

Learn about more wars in history

We hope you enjoyed this look into the wars in history that everyone should know.

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