I'm going to give you an opportunity of an imaginary lifetime -- you can take a two-way trip to anywhere in history, spend thirty minutes there, then return. After this, the time machine breaks. Also, causality, schmausality, anything you change there is now permanently part of history, even if it undermines your existence. (After which you never existed.) So, where in history do we go? Please keep in mind that this is the only time we can do this, as the unobtainium used to power the time machine is literally the only one I could find in the entire world.
I can immediately hear the most popular response -- Kill Hitler.
Yes, German dictator Adolf Hitler can personally be blamed for well over 50 million deaths and was a seriously evil guy. There are worse dictators, (although not many, I can count them on one hand), and he couldn't have done it on his own, so are you sure about this? With the right nudge to history, he might instead be a relatively unknown painter, or even a real estate agent if we change...okay, I can see I'm changing none of your minds. For our thirty minutes in the past, I send a strike team to the eastern front of World War I, between the Russian and Austrian lines. My strike team quickly guns down Corporal Hitler, and returns to the present. So we prevented World War II then? Well...no.
June 18th, 1919
The treaty of Versailles ends World War I with the surrender of the Central Powers. Although the United States lobbies for Wilson's fourteen points, which the central powers would readily agree to, England and France demand harsher teams for the pain they've suffered in the war. The Central powers feel compelled to sign these terms, as they have rather literally run out of soldiers. Nationalists of these nations call bullshit, (as none of the national territory of the central powers actually saw any fighting during the war), but are ignored by everyone else.
Rampant hyperinflation strikes Germany, and to a lesser degree Austria. Even the most basic things require entire wheelbarrows full of money. People become radicalized as they hope for anything that could even possibly relieve their suffering, even for a moment.
An ambition member of the exceedingly fascist Nazi party, Erich Ludendorff, attempts to throw a coup. This fails and he is sentenced to death for treason. The authorities are alarmed at the way that he had over 3000 helpers providing direct help for his plan, and probably had additional agents waiting in the shadows.
The Dawes plan brings some recovery in Germany. Support for extremist parties such as the Communist part of Germany and the Nazi party decreases sharply.
A massive stock crash leads to worldwide economic depression. Well, shit. Hyperinflation returns to Germany, and radical parties now account for at least 45% of the vote in that country.
Japan conquers Manchuria from the local warlord. China protests, but is too disorganized and warlord-riddled to manage an effective response.
President of Germany, Hindenberg, strikes a bargain, with Goering, the leader of the Nazi party, to get them to fight off the communists. This temporarily works well, then badly backfires when...
In a series of "emergencies," Goering increases his powers until having total dictatorial power over Germany. He declares himself to be "Fuerer," or leader, of the entire nation.
Germany and Austria combine into one nation.
Japan and China go to war, with Japan demanding effectively all of China, and China demanding the return of Manchuria.
Goebbles demands the outer portions of Czechoslovakia, the Sudetanland, on the grounds that it's majority German. An international committee of England, France, and other allied countries agrees to allow this. Neville Chamberlain declares "Peace in our time."
About a month after that
Germany absorbs the rest of Czechoslovakia, contrary to previous treaty.
Germany demands that Poland hand over Gdanz, known in German as Danzig. Poland notes its alliance with two world powers, the UK and France, and refuses. War begins in Europe.
France is defeated. The entirety of the French army, and 2/3rds of the British army are captured. The remaining 1/3rd of the British army manages to escape and fortify the UK. Denmark and Norway are quickly conquered by Germany.
Germany declares war on USSR.
Japan attacks Pearl Harbor. USA declares war on Japan, Germany, and the rest of the axis.
Moscow is taken by Germany. Russian government successfully evacuates to Kalomna.
Kalomna is taken. Russian government successfully evacuates to Novosibirsk. Considerable help from the USA is required to keep Russia functional and in the war.
The battle of Stalingrad is won by Russian forces when the Germans retreat. The city is effectively destroyed by the battle.
Second attack on Stalingrad. The ruins are now controlled by Germany.
Maximum extent of the Axis. Germany and their various allies controls the entire European continent from the Atlantic to the Urals, minus the neutral countries of Switzerland, Sweden, and Spain. However, the German high command notes that there are starting to be shortages in manpower, matériel, and armaments.
Stalingrad is retaken by Russian forces. A massive party is thrown in Novosibirsk.
Russian forces retake Moscow. The city is promptly rebuilt, and the Russian command begins to move back. German forces are in full, perpetual retreat.
A British, Canadian, and American army lands on the coasts of France near Normandy. Allied forces slowly push the Axis out of France, and crush the regime in Vichy.
Project Manhattan detonates the first ever atom bomb in a classified test site near Los Alamos, New Mexico.
France is liberated. Charles deGaulle's Free France government assumes control in Paris. United States bombs the Japanese city of Hiroshima, wiping it off the face of the planet. Millions die.
Belgium and the Netherlands are liberated. Their respective governments return from their colonies. Nagasaki is nuked, and millions more die.
Japan surrenders, Korea is liberated. Italy is conquered and under British control.
Russian forces now control Romania, removing it from the axis camp. The city of Innsbruck is nuked, and millions die. Chinese civil war resumes.
Bulgaria is conquered by Russian forces, removing it from the axis camp. Greece is liberated.
Finland switches the the allied side under intense Russian coercion. Hamburg is nuked, killing 90% of its population.
Hungary is conquered by Russian forces, removing it from the axis camp. Germany is now the only surviving axis power, and its days are clearly numbered. Sweden abandons all pro-German sentiment in favor of pro-British, as it is prudent to side with a winner.
Russian forces are at the gates of Berlin. Allied forces liberate the Czech half of Czechoslovakia.
German government decapitated when Berlin is nuked and 90% of the German high command, including Goering, are instantly incinerated.
Russian forces take the remains of Berlin. Czechoslovakia and Austria are now fully liberated.
Germany surrenders, ending World War II. 8 million people died under German imprisonment, and 5% of the earth's entire population died in battle and/or nuclear incineration. The world must not forget. The subsequent party in Russia uses up the country's entire supply of ethanol, starting with vodkas and ending with perfumes.
Huh, that was actually worse. Since we have no more charges on our time machine, we can only hope that the clock-roaches clean up this alternative timeline.