Young people should care about peace more than anybody else. As former president Herbert Hoover said in 1944, “Older men declare war. But it is the youth that must fight and die.”

Hoover was speaking at a time when the country was hoping for peace. During the 30 years since 1914, the United States had fought in two major world wars. More than 500,000 Americans were killed as a result. Elsewhere in the world, over 100 million other human beings also lost their lives.

Many of you, today, may be looking at the wars going on around us. You may be wondering how the world can achieve peace. Can studying history—and in particular the global conflicts of the twentieth century—help us avoid a repetition of the horrors of the past? Can it bring us closer to peace?

In this unit, we approach this question through two kinds of lessons. In some, we seek to understand the causes of three twentieth-century conflicts: the First World War (1914–1918), the Second World War (1939–1945), and the Cold War (1945–1991). In other lessons, we try to describe the human experiences and the terrible cost of these wars.

The First World War

The First World War is a tragic example of a war that didn’t have to happen. It’s a warning about how many small factors can combine to trigger massive destruction. We begin this unit by looking at debates surrounding the origins of the First World War. Was the war caused by the assassination of an Austrian archduke? Or was it the product of trends from the nineteenth century, such as nationalism and imperialism? Or was the war an accident—sparked by a series of bad decisions and miscommunications between countries?

Once we have investigated how the war began, we’ll explore the experiences and outcomes of the war. We’ll look at how a conflict that started in Europe in 1914 became a global war.

Next, we’ll explore the experiences of individuals who fought in and lived through the war. We’ll see how their experiences changed the way they saw the world. We’ll also investigate the very different perspectives of two women: the German antiwar activist Dr. Rosa Luxemburg and the American volunteer nurse Helen Fairchild.

Finally, we’ll consider how whole societies were transformed by the war. We’ll focus on the “lost generation” of young people. We’ll also examine some unanticipated outcomes of the war, such as the genocide of the Armenian people and the Russian Revolution. A genocide is a planned and organized attempt to destroy an entire people.

Interwar

When the First World War ended in 1918, many world leaders hoped to prevent another war. Instead, just two decades later, an even larger and more devastating war broke out. What went wrong? That’s the question we ask in the second portion of this unit, which focuses on the Interwar Period (1919–1939).

We’ll begin by looking at a shared experience that deepened the suffering after the war: the great economic depression of the 1930s. This lesson examines how economies around the world had become much more closely linked. This interconnectedness meant that when one economy collapsed, others soon followed.

In another lesson, we’ll see how movements for international cooperation and peace briefly flourished in the 1920s, but then quickly fizzled out. When they saw how international cooperation had failed to solve the world’s problems, many people lost their faith in democracy. As a result, extreme nationalism emerged once again. This nationalism took many forms, including racism and anti-Semitism. Fascism was the most dangerous of these new types of nationalism. It mixed extreme nationalism and racist beliefs with a call for violence, action, and obedience. Fascists and people with similar authoritarian ideas emerged in many places, including the United States. In a few countries, in particular, Italy, Japan, and Germany, they managed to seize power.

The Second World War

In the 1920s, Japan’s authoritarian government began to seize territory in China and other nearby countries. The international community failed to stop them. In the 1930s, Fascists in Italy and Nazis in Germany invaded their weaker neighbors in Europe and North Africa. Again, nobody was willing to stop them. Germany kept pushing the limits until, in 1939, Britain and France warned Germany not to invade Poland. When Germany did invade, Britain and France declared war, marking the start of the Second World War.

These lessons look at the origins of World War II and the experiences of those who lived through it. In one lesson, we’ll explore how the Second World War began, and how the international community failed to stop it. In another, we’ll look at the human cost of the war in terms of death and destruction. In a third, we’ll look at the experiences of Jewish communities and others targeted by the Nazis in the mass murder known as the Holocaust. Finally, we’ll discuss the dropping of nuclear bombs on the Japanese cities of Hiroshima and Nagasaki in 1945.

Cold War and Decolonization

Sadly, the world didn’t suddenly enter a period of peace following the end of the Second World War. Instead, two new types of conflicts emerged, each linked to the other. One was the global struggle between two nuclear-armed superpowers—the United States and the Soviet Union—and their allies. Because they could destroy each other, these two sides mostly avoided large-scale warfare. Instead, they fought a “Cold War” of smaller conflicts and regional wars. The other type of conflict consisted of wars of decolonization, in which colonies sought to throw off the empires that ruled them. In a series of wars in Cuba, Vietnam, Korea, and elsewhere, these two types of conflicts combined to produce bloody, long-lasting battles.

In these lessons, we’ll look at the causes of the Cold War and decolonization conflicts. In one lesson, we’ll see how the end of the Second World War produced these conflicts. We will then look at the origins of the Cold War and the experiences of people who lived through the arms race and space race that contributed to it. We will also explore communism, and in particular the Chinese Communist Revolution.

In other lessons, we’ll examine decolonization and the different paths it took in various colonies. One lesson focuses on decolonization as it relates to human rights. This lesson addresses women’s roles in decolonization.

The Cold War ended in 1991. But in many ways, the conflicts that are the focus of this unit shaped the world we live in today. It’s worth studying them to see how today’s world was made and to give us ideas about how we might achieve lasting peace in the future.