Napoleon's client kingdom in north-central Germany, created by the July 1807 Treaty of Tilsit for his youngest brother Jérôme Bonaparte and assembled from the Electorate of Hesse-Kassel, Brunswick-Wolfenbüttel, Hildesheim, and other formerly Prussian and minor princely territories. Promoted as 'a model Napoleonic state,' it introduced Germany's first written constitution, the Napoleonic Code, metric standardization, and early Jewish civil emancipation — reforms that outlasted the kingdom itself.
Bankrupted by conscription and taxation for Napoleon's wars, above all the 1812 Russian campaign; dissolved when Prussian forces occupied the kingdom in October 1813 following the Battle of Leipzig. Its territory was redistributed among Prussia, the restored Electorate/later Kingdom of Hanover, and the restored Electorate of Hesse at the 1815 Congress of Vienna.
Dedicated modern English-language monograph on the Kingdom of Westphalia's creation, reforms, and collapse — confirmed on Open Library.