Militarily speaking, the 20th century was dominated by WWI, WWII, and Vietnam. However, between 1900 and 2000, wars claimed the lives of well over 100 million people, many of which occurred in wars other than the aforementioned three. Much of these were smaller-scale, local wars.
In this list, we have compiled six wars from the 20th century that you may not know about.
Russo-Japanese War

The Russo-Japanese War was fought in 1904 and 1905 between, as the name would suggest, the Empire of Japan and the Russian Empire. The two empires clashed when Japan felt threatened by Russia’s goals of expanding its influence into Asia. Although peace talks took place, no agreements could be made and relations quickly broke down.
After the negotiations failed to solve the dispute, the Japanese Navy launched a surprise attack against the Russian Eastern Fleet in Port Arthur, China. Although no clear victor emerged out of this battle, it was a severe knock to Russia’s morale and confidence in their military superiority over Japan. After a series of defeats, Russia retreated into Manchuria, pursued by Japan.
The war cost Japan a larger number of casualties, but they won a decisive victory. Russia and Japan signed the Treaty of Portsmouth, mediated by US President Theodore Roosevelt, which ended the war.
First Congo War

This war is the most recent on this list, and took place between 1996 and 1997, mostly in Zaire. At the time, Zaire had been broken after years of dictatorship and economic struggles. This resulted in a civil war breaking out amongst the different factions within Zaire. While the civil war was taking place, Rwanda invaded, followed by many more African nations. The Zairean forces attempted to fight back but were crushed, and a new government was instated.
Although the war lasted just six months, it claimed hundreds of thousands of lives.
The Winter War

The Winter War was fought between the Soviet Union and Finland in the opening months of WWII. The war would only last for just over three months but had far-reaching effects on the Second World War.
The war began after Finland refused to hand over land to the USSR, which they claimed was needed to safely defend Leningrad. Upon their refusal, the USSR invaded. However, as they would soon find out, Finland was an incredibly tough opponent to fight. The USSR had a major numerical advantage in almost every single way, but the Finns used superior tactics and their familiarity with the environment to overcome the Soviet troops.
Initially, Finland repelled their enemy, but towards the end of the winter, which they had used to their advantage, the USSR reorganized their forces with renewed tactics. As a result, they were able to force a peace treaty. Their valiant and unexpected resistance against the USSR increased Finland’s international reputation. The USSR would later use tactics learned from the Finns against the Germans.
The First Indochina War

The First Indochina War took place between 1946 and 1954 and set the groundwork for the Vietnam War. During the war, French forces battled the Việt Minh but were eventually defeated. Vietnam had been a French colony until 1940 when Japan occupied the country. When Japan surrendered in August 1945 Vietnam experienced a power vacuum, which was quickly filled by Ho Chi Minh. However, because of Ho Chi Minh’s communist allegiances, France attempted to reestablish control over the country.
In 1946 negotiations between France and the Việt Minh government broke down and the two clashed in combat. The war ended in 1954, shortly after France’s crippling defeat at Dien Bien Phu. After the war Vietnam was split in two, with the Democratic Republic of Vietnam in the north and the Republic of Vietnam in the south.
Chaco War

This war was fought from September 1932 to June 1935 between Bolivia and Paraguay. The two South American nations battled for control over an area of Gran Chaco after oil was discovered there. After this, Bolivia became interested in extracting oil from the region and accessing the Paraguay River, which led into the Atlantic Ocean.
Paraguay had already lost vast amounts of territory in the late 1800s, and, unwilling to lose anymore, they fought hard throughout the war. Bolivia was better equipped, but poor morale and a lack of motivation amongst troops meant Paraguay was able to get the upper hand. The entire country gathered to aid the war effort in any way possible.
Both sides did not possess their own armaments industries and relied on foreign shipments to arm their troops. In addition, they were both severely affected by a lack of supplies and any substantial communication networks. Despite this, Paraguay emerged as the victor.
Sino-Indian War

On October 20 1962 Chinese forces crossed a large section of a disputed border with India in the Himalayas, prompting a war between the two nations. The war lasted just a month and one day, and was fought in brutal conditions high up in the Himalayas. Chinese troops outclassed India’s less numerous forces, forcing India to make a number of retreats.
China announced a cease-fire on November 20, ending the war. Chinese troops then pulled back to their “Line of Actual Control.”
The war showed India that changes were sorely needed if they wanted a more capable military, resulting in them doubling their manpower over the next two years.