All wars are awful. Some wars are much, much more awful than others.
This is by no means a comprehensive list, nor does it comprise anything but a fraction of the overall deaths in wars in human history. Still, the five wars on this list may have collectively killed up to a quarter of a billion people.
These wars were big and upset the status quo. The Chinese Civil War turned more than half a billion people Red. World War II destroyed a totalitarian menace. Even the Mongol invasions echo in the present as an estimated 16 million people worldwide  carry the genes of Genghis Khan.
Chinese Civil War
The Chinese Civil War was fought between the forces of the Republic of China (ROC) and the Chinese Communist Party (CCP). The war was fought on and off for more than 20 years, from 1927 to 1950, and resulted in the establishment of the People’s Republic of China on the mainland and the Republic of China on the island of Taiwan. Some eight million were killed in a conflict complicated by the presence of Japanese forces in China.
Like all civil wars in China’s history, social disruption was the main killer and affected civilians the most. Fighting generated refugees, leaving them vulnerable to disease and starvation. Reprisals by one side against cities, towns and villages thought to be sympathetic to the other killed more civilians.
Military casualties in the beginning of the civil war were relatively light, as the CCP primarily fought a guerrilla war. At the end of World War II the Soviet Army provided captured Japanese weapons to the CCP’s military forces, dramatically increasing their effectiveness in the field. Within five years the ROC had been swept from China into Taiwan and pockets of Southeast Asia.
An exacerbating factor in the civil war was the presence of Japanese forces engaged in a brutal campaign to pacify occupied China. The Japanese were usually more than a match for Chinese forces, but China had a seemingly inexhaustible amount of manpower. Both ROC and CCP forces fought the Japanese, even temporarily suspending fighting one another during the famous Second United Front .
Tai Ping Rebellion
Hong Xiuquan , a Chinese Christian mystic who believed he was a brother to Jesus, led a revolt against the ruling Qing dynasty. Hong founded the Tai Ping Heavenly Kingdom, and led an army to overthrow the Qing. The civil war, which lasted from 1850 to 1864, was possibly the most lethal conflict ever.
Hong’s rebellion started in southern China, with many of its recruits coming from Guangxi and Guangzhou provinces. As the Taiping Heavenly Kingdom marched north, enjoying victory after victory over Qing forces, a capital was set up in Nanjing.
The advance of the Taiping Army was halted by the Ever Victorious Army, an Imperial army led by European officers, including American Frederick Townsend Ward and British Army officer Charles “Chinese” Gordon , who would later be killed at the Siege of Khartoun. The Taiping Army proved unable to capture Beijing and Shanghai, and was eventually rolled back by Imperial forces.
Although military casualties were likely under 400,000, total casualties including civilians were reportedly anywhere from 20,000,000 to 100,000,000. Most civilian casualties were caused by civil disorder and resulting starvation and disease. Towards the end of the war Imperial government troops conducted reprisals in the birthplace of the rebellion, with up to one million killed in Guangzhou.
Mongol Conquests and Invasions
The Mongols, a tribe of nomadic horsemen from Central Asia, conducted a hundred year campaign of conquest that subjugated most of Eurasia. During the 13th century, the Mongol Empire systematically conquered modern-day Russia, China, Burma, Korea, all of Central Asia, India, Iran, Iraq, Turkey, Bulgaria, Hungary and Poland.
The Mongols did not conquer gently. Between 1211 and 1337, they may have killed as many as 18.4 million people in East Asia alone. As Ian Frazier wrote  in The New Yorker, “For the cities and cultivated places in the Mongols’ path, they were a natural disaster on the order of an asteroid collision.”
An example of Mongol brutality was the Persian city of Nishapur, destroyed in 1221 AD by Mongol forces who reportedly wiped out 1.7 million people living in and around the city. In their conquest of Baghdad, then the capital of the Abbasid Caliphate, the Mongols embarked on a seven-day killing spree that killed 200,000 to 1,000,000 inhabitants of the city.
Exactly how many people were killed in the various Mongol invasions is difficult to pin down. Historians have likely exaggerated many of the statistics, helped by the Mongols themselves. The Mongols spread word of atrocities far and wide to demoralize those next in line for conquest. Revisionist studies of the Mongol invasions have proposed rolling back the number killed considerably, from roughly 40,000,000 to perhaps “only” 11.5 million  during a period of 120 years.
World War I
Sixteen million people were killed in World War. Of those, 9,000,000 were combatants and 7,000,000 were noncombatants.
The high death rate in World War I was a result of several factors. Political demands dictated every square foot of national territory must be held, which necessitated large armies. Militarily, many armies maintained an unflinching attitude towards maintaining the offensive, despite the fact that — for the time being — the defense was stronger than the offense.
World War I was the first Industrial Age war fought on a global scale, introducing machine guns, tanks and artillery on a widespread basis. The machine gun in particular dramatically increased levels of firepower for the infantry—but mostly in the defense.
World War I was marked by several grinding, bloody battles that became infamous for losses incurred on both sides. One of the first was the First Battle of the Marne, which saw French casualties of 250,000. Germany’s casualties are only an estimate but thought to be equal to those of the French.
The First Battle of the Marne, rather than repulsing military and political leaders and forcing them to change tactics, merely set the tone for the rest of the war. The Battle of Verdun cost an estimated 714,000 casualties during a three hundred day period. Total casualties at the Battle of the Somme are thought to be between 700,000 and 1.1 million. Casualties on the Eastern Front were worse, with 300,000 Germans and 2.4 million Russians killed—many due not to not combat but hardship and disease.
World War I was also probably the last time a war with a large death toll claimed more combatant lives than noncombatants. Despite so much of the war being fought on French soil, French civilian deaths are thought to be only 40,000.
World War II
The most lethal war in human history is almost certainly World War II. Other wars may have been more lethal but lack credible records. Sixty to eighty million people died between 1939 and 1945. Twenty one to twenty five million of the deaths were military, the remainder civilian.
The concept of Total War, in which the scope of legitimate wartime targets is extended from the enemy military to the state supporting it, relaxed previous restrictions and made even cities targets. Strategic bombing allowed air forces to drop bombs deep behind enemy lines, and civilian deaths from aerial bombing reached at least one million.
Unlike World War I, World War II was a truly global war with much of the fighting taking place in Asia and the Pacific. The Soviet Union lost an estimated 27 million military personnel and civilians, making it by far the country with the highest death toll. China is thought to have suffered 20 million deaths, Germany 6-7 million, and Japan roughly 2.5 to 3.2 million. The United States was fortunate, losing approximately 420,000, all but 10,000 military deaths.
Further exacerbating the number of civilian casualties was the large amounts of territory occupied by the Axis powers. Germany and Japan were both brutal occupiers, and civilians in countries such as Yugoslavia, the Soviet Union, Poland, China and the Philippines—just to name a few—suffered appallingly.
Acts of genocide contributed significantly to the war’s death toll. Germany’s campaign of extermination against Jews, Slavs, Roma, homosexuals, German dissidents and the disabled claimed an estimated 11 million lives.
Kyle Mizokami is a writer based in San Francisco who has appeared in The Diplomat, Foreign Policy, War is Boring and The Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch.