Some countries that no longer exist
Here are some:
The Republic of Salò or the Italian Social Republic was a German puppet state recognized by Germany, Japan and a handful of other countries that existed from September 1943 to May 1945. In July 1943, the Allied forces pushed Italy out of North Africa and Mussolini was arrested. Germany quickly intervened and seized control of the northern half of Italy, freed Mussolini and brought him to the German-occupied area to establish the Republic of Salo. The state was centered on Salò (hence its colloquial name), a small town on Lake Garda, near Brescia, where Mussolini and the Ministry of Foreign Affairs were headquartered. The state came to an end with the surrender of German troops.
Neutral Moresnet was a small Belgian–Prussian condominium (between present day Netherlands, Belgium and Germany) that existed from 1816 to 1920 and was jointly administered by the United Kingdom of the Netherlands and the Kingdom of Prussia. Belgium took over from Netherlands after 1830. When the map of Europe was redrawn at the Congress of Vienna in 1814–15, United Kingdom of the Netherlands and the Kingdom of Prussia both laid claim to the area of Moresnet because of zinc mine in the area. An agreement was finally reached that Moresnet would be jointly administered. The territory is now a part of Belgium.
Czechoslovakia existed as a single country from October 1918, when it declared its independence from the Austro-Hungarian Empire, until its peaceful dissolution into the Czech Republic and Slovakia on 1 January 1993.
From 1958 until 1961, the United Arab Republic (UAR) was a political union between Egypt and Syria. It was a step towards unification of the two countries to deter communism and unite the Arab world. Syria was not pleased with Egyptian policies. A coup took place in September 1961 and UAR ceased to exist.
Yugoslavia came into existence in 1918 after World War I. Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia, and Slovenia were the six constituent republics of Yugoslavia and Vojvodina and Kosovo were two autonomous province. Croatia and Slovenia declared independence in 1991. Further break-up followed soon.
East Germany existed from 1949 to 1990 until the unification of Germany.
The Soviet Union (USSR) existed from 1922 to 1991.