While Germany is moving westward to attack France and Britain, Russia begins to mobilize to Germany and effectively splitting their forces. While not for long Russia likely weakened Germany enough to make a difference
Sinking of the Lusitania
When German forces moved west to attack France the two met at the border and employed a new tactic. Both sides created trenches and hid inside them in dangerous and dirt conditions for almost 5 years.
Assassination of the Duke Franz Ferdinand
The invasion of Belgium is the German march through Belgium to get to France. Belgium had remained a neutral nation and the march had compromised that. Belgium's alliance with Britain requires them to defend their neutrality and join the Western front.
The Lusitania was a British trade ship carrying many people from different countries. The Lusitania was attacked and sunk by German U-Boats, killing 128 Americans, and with the help of the Zimmerman note pulls the U.S into World War 1.
While Franz Ferdinand was visiting one of the countries he rules over he was assassinated by Serbian agents. This pushed Austria to declare war on Serbia, pulling Germany into the war.
On November 11th 1918, the European forces agreed to end fighting on the western front. These agreements led to the creation of the Treaty of Versailles which ended WWI but did not prevent WW2.