A TL I thought through quite well and it is on my favourite time period... World War One.
Assassination at Sarajevo
Archduke Francis Ferdinand was brutally murdered on June 28th by an Austro-Hungarian citizen and Serb Nationalist, Gavrilo Princip. The enraged Austrians sent an ultimatium to Serbia in hopes for an easy gain on territory. Germany also decided to give it's support to Austria.
"Big Brother" Steps In
Russia considered itself the "Big Brother" of the Slavic Nations and began mobilization against Austria-Hungary and Germany. Since France didn't want to face Germany alone, and they wanted to help an ally of the Entente. Britian makes it known that it is neutral and will step in if there is a threat to another neutral nation.
Secret Ottoman Alliance
In a secret alliance with Germany, Ottoman Turkey officially declares neutrality. With this move the German Kaiser was slightly angered, but he was able to get the Sultan's cabinent to sign it irregardless, allowing the Ottoman Empire to join the Central Powers.
Early Western Front
With the Great War off to a start German commanders oddly decide it would be best to wear out the French troops and set up skirmishing posts along Aslace-Lorraine as they set up trenches and forts along German territory. Germans get lucky as the French excpect a German attack and prepare their own defence, then they realize their enemy is doing the same and attacked through Aslace-Lorraine as expected. With more time to prepare than expected, Aslace-Lorraine falls and the French see fierce resistance slowing progress to a halt.
Early Russian Front
Germany and Austria thought they could wait to send troops to the Eastern front, but Russia began invading East Prussia and sieged Krakow. With historic German territory under invasion, many German people quickly join the German Army to defend their homes. Russia doesn't get very far into Germany before bieng pushed back into Greater Poland.
Early Balkan Front
Serbia is invaded and nearly gets completely occupied before large resistance takes a heavy toll on Austrian supply lines, although thier capital is taken, they refuse surrender and continue the fight.