Short answers, pulled from the story.
Revanchism is a political doctrine aimed at reversing territorial losses through revenge or reclamation. The term originated in 1870s France after the Franco-Prussian War, when nationalists demanded the return of Alsace and Lorraine lost to Germany.
French revanchism drove France to court the Russian Empire, producing the Franco-Russian Alliance of 1894 and eventually the Triple Entente of France, Great Britain, and Russia that fought World War I. The ultranationalist tradition shaped French foreign policy from the 1870s through at least 1921.
The Treaty of Versailles of 1919 restored Alsace-Lorraine to France and extracted reparations from Germany. The conference opened on the anniversary of the proclamation of the German Empire, and Germany was required to sign the treaty in the Hall of Mirrors, the same room where that empire had been proclaimed.
The Trianon trauma refers to the deep national grief over the Treaty of Trianon of 1920, which reduced Hungary significantly and left large Hungarian populations outside its new borders. A study conducted in 2020 found that two-thirds of Hungarians agreed parts of neighbouring countries should belong to Hungary.
Pan-Germanists in the Weimar Republic demanded the reincorporation of Alsace-Lorraine, the Polish Corridor, and the Sudetenland. Adolf Hitler supported those claims, and the drive to recover them led directly to the invasion of Poland and the outbreak of World War II.
Russia's annexation of Crimea in April 2014 and its invasion of Ukraine in 2022 have been widely described as revanchist by Western political observers. Some Russian nationalists also consider Alaska, sold to the United States by Russia in 1867, to be territory that should be returned.