Buildup to war
Germany had formed as the world's greatest power, feared by even the mighty British Empire. The Central Powers consisted of Germany, Austria, Italy and Denmark (a puppet of Germany in this timeline). Germany had the military, Austria had the loyalty, Italy had territorial claims on France and Denmark had the navy and was a puppet/vassal of Germany. This alliance was directly opposed to the Allies, who consisted of the British Empire (including Dominions), France, Japan (although isolated from Europe) and Belgium (in British sphere). Russia is to busy working on Reforms to join either alliance and is also to primitive and unstable. The alliances are becoming more and more hostile towards each other in the early 20th century.
An Archduke Assassinated & The Beginning of War (1914-1915)
On June 28th, 1914 a young Serb patriot kills Archduke Franz Ferdinand, his wife and 20 random civilians who are caught in the middle of the gunfight between the patriot and the Archduke's guards. The guards eventually kill the young man, but it is to late; Archduke Franz Ferdinand bleeds out a few hours later, his wife soon follows. Franz Joseph declares war on Serbia and dies just a few days later. Charles I takes over and asks Wilhelm to send 25,000 German troops to assist in the conquest of Serbia; Wilhelm accepts. Austria invades and soon after Annexes Serbia. Germany invades the small country of Luxembourg and annexes it just days later, preparing for the Schlieffen Plan; all they need is France to declare war.
World War 1 (1914-1920)
France declares war using the excuse of retaking Alsace-Lorraine and liberating Luxembourg. Germany has rapid victories at first and annex Belgium in mere months. Italy officially joins the war along with Austria after German victory at Brussels. Italian armies are pushed back over 100 miles initially, however a swift counter-offensive is launched and successful as Austria loans 50,000 troops as support. A German military officer invents Blitzkrieg and informs Wilhelm of the strategy; Wilhelm implements it as an experiment at the battle of Lyon, in which both Germany and Italy's troops use the tactic; it is wildly successful. With the new Blitzkrieg tactic Germany approaches and soon surrounds Paris in late February 1915. France is shocked at the rapid loss of life and land and surrenders to Germany on March 2nd, 1915 with the treaty of Paris. The treaty says France may not have more than 50,000 active troops and 50 naval ships and no troops can be near the German or Italian border, Italy gains the area around and including Nice, Germany doesn't want any land. The British forces in northern Africa attack Italian Libya, but are expelled soon after as Germany's 13th Division of Africa supports Italian troops in Libya. An Italian counter-attack fails also, however, and Italy just focuses on defending the colony. Japan officially leaves the Allies in 1915 as they do not care about the war. The British Empire continues fighting the war until 1920, but not much happens between the warring nations except small skirmishes in the north of Africa.
End of the war (1920)
The British ask for peace in 1920 as not much has happened and Germany agrees, leading to the (first) Treaty of London. The British pay war reparations (500,000 Dollars), but that's all. Germany formally annexes Belgium (including Belgian Congo which becomes German Congo) and the British recognize German annexation of Luxembourg. The war is over. The Dominion of Canada is at the treaty and asks the British for full independence, to which Britain reluctantly accepts.