While it is often tempting for patriotic people to think of their homeland as permanent and everlasting, it is an undeniable fact that sometimes even countries cease to exist.
There are several possible reasons for a nation to come to an end. Some countries merge to form (or reform, in the case of East Germany and West Germany) a single country. Other countries split apart—for example, the Union of Soviet Socialist Republics (U.S.S.R.), dissolved into 15 smaller countries in the late 1980s and early 1990s. Some countries have been annexed, such as when the growing United States of America absorbed the fledgeling Republic of Texas in 1845. Others have been conquered outright and assimilated, as happened when Vietnam absorbed the kingdom of Champa. Finally (and least catastrophically), some nations have remained intact and simply adopted a new name, such as when Ceylon "died" and was reborn as Sri Lanka in 1972.
World War I and World War II saw immense disruption to the alignment and existence of many countries throughout the world. Alsace-Lorraine, the region that straddles France and Germany, ceased to exist in 1918 with the end of World War I. The Weimar Republic became Nazi Germany, both of which no longer exist and are parts of the modern country of Germany.
The slow-motion collapse of the Soviet Union from 1988-1992 led to the introduction of 15 separate countries in Europe and Asia. Czechoslovakia and Yugoslavia, two countries that emerged out of World War I and World War II and which had strong ties to the U.S.S.R., dissolved alongside it. Czechoslovakia became the Czech Republic and Slovakia, and Yugoslavia broke apart to form Bosnia and Herzegovina, Croatia, Montenegro, Macedonia, Slovenia, Serbia, and Kosovo (which was struggling to attain independence from Serbia in 2021).
In the Middle East, The United Arab Republic was formed in 1958 by merging Egypt and Syria; the UAR then attempted to add a third country, then called North Yemen, and become the United Arab States. Both the UAR and UAS dissolved in 1961.
Some view the Native Americans and First Nations as having historical sovereign states, as they once had their own governments and tribal alliances, but are now part of modern countries such as the United States, Canada, and Mexico. There is significant contention around this idea, as the US Constitution allows for autonomy of the native tribes, but this autonomy is often poorly respected or simply ignored.
The countries that no longer exist are as follows: Abyssinia, Austria-Hungary, Basutoland, Bengal, Burma, Catalonia, Ceylon, Champa, Corsica, East Germany, East Pakistan, Gran Colombia, Hawaii, New Grenada, Newfoundland, North Yemen, the Ottoman Empire, Persia, Prussia, Rhodesia, Siam, Sikkim, South Vietnam, South Yemen, Southwest Africa, Tanganyika, Texas, Tibet, Transjordan, the USSR, the United Arab Republic, Vermont, the Republic of West Florida, England, Scotland, Wales, West Germany, Western Samoa, Yugoslavia, Zaire, and Zanzibar.