Prinz Wilhelm, after whom the first railroad was named, accepted the titles of Saxony, Finland, and Poland, after the heirs died. He was crowned, and promptly did away them all. No longer were Prussia, Warsaw, Finland, or Saxony separate entities. Instead, Wilhelm merged them all and created the Nordreich, or Northern Empire, becoming Kaiser Wilhelm I. All the areas within the Nordreich were granted a level of autonomy and rights, and Kaiser Wilhelm hoped to bring Prussia streaming into an industrial future, wealthy beyond imagination.
Napoleon II was very swift in reacting to these events, as he saw the creation of a "Northern Empire" as a direct threat to his own Empire. His first telegrams with Wilhelm concerned the French peacekeepers present in Finland. Wilhelm calmed him and told him they could stay, as "Prussia never dishonors treaties." They did agree to negotiate on an eventual withdraw, though it wouldn't happen till decades later. Napoleon II grudgingly accepted Wilhelm's rule and formally recognized his legitimacy.
The Prussians and the French got involved in the "Great Game" in which each side vied for colonial possessions worldwide. In the Race for Africa, Prussia acquired the vast Congo regions. Later in the 1900s, the Prussians also founded the Central Powers, a military alliance between the Britannic Union, Norway, Sweden, and the Netherlands. They were the primary combatant in the Great War.
During the Versailles Peace Treaty, Prussia received most of Northern Germany from the former First Rheinbund. After much instability plaguing the country, the Prussian Empire collapsed and the Germanic Civil War began. By the 1920s, the Prussian Empire was abolished and was succeeded by other countries. Hohenzollern royal family members fled to Prussian African possessions and declared the "Mittelafrikan Reich".