A member of the Consulate government that emerged sometime after the French Revolution Napoleon I soon took dictatorial control of France, and was officially declared Emperor in 1804. At this point Napoleon has already annexed Belgium, Holland, and has control of even more territory in Europe.
In 1806 when Napoleon established the Berlin Decree and the Continental System King George II embraced it, and with the support of Prime Minister Jefferson was able to get Parliament to ban trade with Great Britain. Instead of the Embargo Act in OTL that stopped all trade the embargo was only against Britain. As a result nations in Europe with the exception of Portugal accepted the continental system.
In 1807 Napoleon invaded Portugal and Spain. The following year the American navy in Northern Africa entered Spain in order to help Napoleon Bonaparte. As a result Great Britain declares war on America. Unlike in OTL this ends with a French victory. Spain and Portugal are partitioned between France, which gets most of the peninsula and America which gets the Balearic Islands, and the south of Portugal and Spain.
In 1808 Napoleon reinvaded Haiti, which had been lost to the Haitian Revolution, and paid the United States to lend it military support. King George II accepted, and would buy Haiti from France the following year for double the amount. The following year King George II gave verbal support to Napoleon's decision to annex the Papal States. This appealed to popular anti-Catholic sentiment at the time, even though France was still mostly Catholic. In the meantime Napoleon's armies were marching across Europe conquering vast swaths of land.
Since Russia does not exit the Continental System Napoleon is free to focus on conquering England, while the United States fights a war closer to home. The United States invades Canada. In 1814 after years of fighting France wins a decisive victory over England. A treaty is signed handing Canada over to the United States, while Britain comes under French rule.
At this point Napoleon controls all of Europe west of a shrunken Prussia and the newly formed Austrian Empire except for Scandinavia and Denmark, and Southern Iberia. On the eastern borders is Prussia, Russia, and the Austrian Empire. The Duchy of Warsaw (Poland) is independent. A number of autonomous kingdoms under the rule of the French Empire are recognized. In 1816 he renames the Empire the European Empire and even appoints a King of France underneath him in order to diffuse nationalist tensions. Most colonies of these European powers pass to Napoleon. However, Mexico continues in its revolution, and Venezuela remains independent as well. Brazil is not annexed and is officially still under the Portuguese House of Bragança, which does not recognize Emperor Napoleon or King George II's sovereignty over their respective parts of Portugal. Sweden has been absorbed by the Kingdom of Denmark-Norway and Finland has been conquered by Russia. Moldavia had also been completely annexed by Russia.
Napoleon does not stop. His proclamation of the European Empire appeals to nationalist sentiments of ethnic minorities in the Austrian Empire. In 1817 this leads to the war of the 8th Coalition. Napoleon invades the Austrian Empire promising kingdoms and duchies considerably more autonomy than they are given by the Austrian Emperor Franz I. Prussia, a now-small kingdom is promised control of a considerable portion of the Confederation of the Rhine if it helps Austria, so it joins. Russia, fearing it might be next aligns against the European Empire. The Kingdom of Hungary also joins. The Kingdom of Denmark-Norway joins the war on the side of the Coalition, also fearing eventual invasion in 1821 after Napoleon's death. The United States, while instrumental in helping Napoleon win previous wars decides to take a stance of neutrality towards this war.
Prussia is quickly overrun, and the King of Prussia goes into exile in the Austrian Empire. In 1818 the following year the Duchy of Warsaw joins the European Empire in exchange for promised land in Russia. The Austrian Empire is powerful, and the war still is not finished at his death in 1821. However, he does succeed in taking Vienna in 1821 shortly before dying, forcing the royal family to evacuate inland.