Here is an very simplistic and dumbed down response, with no regard for the true complexity of 20th century military history: PT1 -WW1: The great war starts in 1914, the murder of archduke Franze Ferdinand of Austria by a Serbian terrorist causes a war between Serbia and Austro-Hungary. Russia and France join in support of Serbia, Germany joins in support of Austro-Hungary. England and friends (Canada, India, Australia) join against Germany when Germany invades Belgium as a corridor to invade France. Ottoman empire (Turkey) joins Germany and Austria-Hungary.

Germany makes good progress at first and is stopped and forced away just outside Paris, the war turns into a bloody stalemate with almost unimaginable casualties, thus far unheard of in human history. Russia collapses as a result of its failure against Germany and Austria and is taken over by communists and US joins England+France. Germany Surrenders in 1918 with Humiliating Terms, Austro-Hungarian empire and Ottoman empire break up.

• WW2: In 1933 Germany, facing economic depression elects a Facist government led by Adolf Hitler, who rapidly builds up Germanies economy, industry and military (the latter in secret as WW1 surrender terms forbade it) all the while eroding Germanies democracy and enacting policies against "undersirables" (Jews, Homosexuals, Communists, Gypsies)

Meanwhile Japan has been rapidly building an empire in Asia, occupying Parts of China and southeast asia, USA imposes sanctions to slow expansion.

In Spain, the Facist army of Francisco Franco overthrows the Government of Spain in a brutal civil war. The Soviet Union (Russia) and international volunteers supported the Government, while Germany and Italy supported Franco.

-Germany begins annexing parts of its old empire in Europe. Britan and France finally draw the line at Poland. Hitler (dictator in Germany) makes a secret pact with Stalin (dictator in the Soviet Union). In 1939 Germany invades Poland from the west and crushes its military with ease, Britan and France declare war on germany but do little to help. The Soviets invade Poland from the east, encountering almost no resistance.

Germany then rips through western Europe, finally invading France in 1940. The French army collapses and the government surrenders in a few months. British troops retreat to England.

Germany is unable to invade England and so invades the Soviet Union in 1941. Once again moving with ease until being stopped by Soviet troops just outside Moscow with an extremely cold winter rolling in. Germany resumes its invasion the next summer, pushing south this time. Finally stopped by stubborn Soviet troops in a city called Stalingrad in arguably the bloodiest battle ever. The German force is surrounded and destroyed that winter and Germany begins a long, bloody retreat through Russia and east Europe back to Germany, fighting massive battles along the way.

In December 1941 Japan cripples the US navy at pearl harbor in a suprise attack, the US declares war (and Germany declares war on the US) but America quickly loses much of its pacific holdings to Japan, before the Japanese navy is badly beaten near the island of Midway. The US begins a campaign of pushing Japan back one island at a time.

Meanwhile, British forces in North Africa have been kicking the crap out of the Italian army, before being forced back with the arrival of German troops. A back and forth war of attrition results, with both sides pushing eachother across the desert. USA joins the campaign, and the US-UK Allies finally evicts the Germans and Italians from North Africa.

The Allies then invade Italy, the Italian military collapses but the Allies make slow progress against the Germans deployed there.

The Allies then invade German controlled France in 1944, slowly pushing Germany back from the west, while the Soviets have been attacking from the East. The US and British airforces have also been bombing German cities to rubble. Soviet troops take the capital city (Berlin), Hitler commits suicide and Germany surrenders in 1945, the Allies and Soviets discover slave camps and death camps where Germany has been killing Jews, Gypsies, homosexuals and political dissidents in the millions.

Back in Asia, the US has destroyed Japans navy and now has control of islands in bomber range of Japan itself. Instead of a costly invasion the US deploys its newest weapon and destroys the cities of Hiroshima and Nagasaki with atomic bombs. The Soviets also move troops who have just beaten Germany to asia, and easily crushes the Japanese army in that had been fighting the Chinese in northern China.

As a result of these events, Japan surrenders in 1945.

Last 100 years of war for dummies PT2:

The USA and Soviets (USSR) are now the most powerful nations on earth, the USA has a massive military, strong economy and nuclear weapons, and the USSR has the largest and most powerful army ever assembled. Germany has been split between the areas controlled by the Allies and the areas controlled by the Soviets. Mistrust and ideological differences between the two force most of the world into allying with one or the other, with England, France, West Germany, Japan supporting the USA, and Poland, East Germany, and (now communist controlled) China supporting the USSR. The USSR quickly builds up its own nuclear stockpiles.

The two superpowers and their major allies avoid direct confronation, as each possess extremely powerful armies and enough nuclear warheads to destroy the other utterly. However they fight by proxy, as many countries that were occupied by Japan in WW2 are split in half between US and USSR influence.

Korean war: In 1950 the USSR backed, communist North Korea invades the capitalist, USA backed south. The North Koreans nearly succeed but are forced back and nearly defeated when a coalition of nations, mainly US forces enter the Korean war. As North Korea nears defeat the Chinese move to support them and push the allies back to near where the old border was. A fragile ceasfire is reached that persists to this day, with occasional flare ups of violence between the two.

Vietnam war: In Vietnam, France attempts to regain control of its old colony but is defeated by Vietnamese communists. Vietnam, much like Korea is split between Capitalist, US backed south and Communist, USSR backed north. When elections to reunify the country are halted by the south, the communist north invades in 1959.

Eventually the US is drawn into the war, and while they are usually successful against the North Vietnamese army and pro communist insurgents combat, they are unable to stamp out resistance. The steady trickle of Americans casualties and deaths of large numbers of Vietnamese civilians causes the war to become unpopular among the US population. The US finally withdraws in 1973, and the North quickly overruns the south.

(moving back in time, to a different part of the world) In 1948, in the British controlled middle east the nation of Israel is formed to establish a home country for the Jews. This is met with resistance by the Muslim population in the area. The Jewish Iraelis come out on top over the Muslim Palestinians in a civil war. The middle east becomes another area of US-Soviet tension, with the US and UK supporting Israel, and USSR supporting the Arab nations with weapons and political support.

Six day war: In 1967 the six day war begins. Isreal launches a pre-emptive attack against its more powerful neighbors of Egypt, Syria and Jordan in response to their military buildup. The airforces of the Arab countries are destroyed, and Isreal quickly defeats the armies of all three, taking large swaths of land in the process.

Yom Kippur war: In 1973 Egypt and Syria launch a suprise attack on Israel in whats called the Yom-Kippur war. Isreal did not pre empt this time and the Arabs regain much of the land lost in the six day war. Israel finally manages to turn the war in its favor and a cease fire is reached.

In 1979 the US backed Shah of Iran is overthrown in a revolution and the country becomes a radically Islamic (shia muslim), anti US nation.

Iran-Iraq war: In 1980 Iraq (controlled by a sunni muslim dictator named Saddamn Hussian) invades Iran as a result of border disputes and fears the revolution would spread to the oppressed shia majority in Iraq. Iraq is indirectly supported by the US and its military is superior to the Iranian army, which was gutted by the revolution. However Iran has vast manpower reserves and pushes Iraq back in long, bloody war of attrition before peace is finally reached in 1988, with no major gains of territory on either side.

Gulf war: In 1990 Iraq invades and quickly conquers the small nation of Kuwait, and fears that they could invade Saudi Arabia and control much of the earths oil supply causes the US, England, France and an alliance of Arab countries to move to defend Saudi Arabia. After the UN deadline for Iraqs withdrawel expires the US and UK begin a bombing campaign to destroy Iraqs military, as well as its chemical and nuclear weapon production and development.

In 1991, after a small Iraqi raid into Saudi Arabia the ground war begins the the US/UK led allies easily smash the Iraqi military in a one sided campaign and liberate kuwait in only 100 hours.

Now I'm tired of typing, but other major areas are: Constant wars between India and Pakistan, Complicated wars in the Balkans that will make you lose your faith in humanity, endless african wars that are somehow even worse than the balkans, and the Faklands war between England and Argentinia.