Final answer:
The Treaty of Trianon dealt with Hungary, reducing its territory and significantly altering its borders after World War I.
Step-by-step explanation:
The country dealt with by the Treaty of Trianon was Hungary. The Treaty of Trianon was signed on June 4, 1920, as part of the peace settlement that concluded World War I. Following the war, the Austro-Hungarian Empire collapsed, leading to the establishment of several independent nations. The Treaty of Trianon specifically addressed the territorial adjustments necessary to formalize Hungary's reduced size and borders after the disintegration of the empire.
As a result of the treaty, Hungary lost about two-thirds of its territory and about one-third of its ethnically Hungarian population to the neighboring countries of Austria, Czechoslovakia, Romania, and Yugoslavia. This territorial loss was a significant point of resentment for Hungarians and a contributing factor to the tensions that preceded World War II. The borders established by the treaty are largely upheld in modern Europe, despite the changes that took place after the Second World War and the subsequent redrawing of Europe's political landscape.