A vast majority of the world. Too broad to cover in 100% detail, but let's do highlights of the big ones:
Founded by conquest of Kievan Rus slavs originally (double extra dose of irony: original Kievan Rus territory is now outside of Russia; in Ukraine). Later; founded as independent state by a war of independence against Mongols in 14th century. Large parts of modern day Russia were conquered by force of arms in 15-19th centuries (Central Asia, near-Caucasus republics, North-West from Poland/Lithuania/Sweden as random examples).
Early Chinese states were all founded through violence. Modern PRC and ROC were founded as a result of civil war between Mao and CKS in 1940s.
War of Independence against Britain; and armed conquest of large chunks of territory (Texas from Mexico; Florida; arguably many areas from Native American tribes during 19th century. Mind you, before yelling about whites, those samesuch Native American tribes conquered the area previously - the most ironic example being the stereotypical case of complaints, the Black Hills of Dakota, which is usually portrayed as US government conquering from Lakota/Sioux... forgetting to mention that they conquered the area from previous tribes a hundred years prior).
Originally who knows. Then, Mexica conquered it and built their empire. Then Spanish conquered it and made a bunch of Central and South American colonies. Then there were wars of independence against Spain.
Similar was the case with many other South/Central American states, except in places replace Spain with Portugal.
Almost all of Europe
Most of Europe is a result of (1) conquest of steppe tribes; (2) conquest of - and genocide of - Celts by Roman Empire, starting with Ceasar; (3) More recently, assorted wars and invasions of everyone against everyone, including but not limited to Turkey/Ottomans/Arabs in south-eastern Europe.
For specific modern countries you also have: Italy was war of Independence, France was 100 year war and other wars, Spain and Portugal was Reconquista, Austria was constant war against Turks, Germany is hard to pin but I'll go with 30 year war (ditto Netherlands); Poland in the most recent case WWII and prior to that tons of wars including those involving Swedes and Russians.
Most of Middle East
Arab revolts against Ottomans in early 20th Century in most recent cases. Of course in case of Persia/Iran and Iraq/Assyria, you have gazillion of years of warfare and invasion before then, starting with Hittites, Babylon, Assyrians, Persians, etc...
Egypt had invasions in ancient history which broadly formed it, then Alexander the Great that formed a more modern Egypt under Ptolemeic dynasty.
Turkey was most recently formed through conquest by Ottomans (and earlier conquests); with Armenian genocide in early 20th century thrown in.
Since the context of asking the question is likely USA's recognition of Jerusalem, let's dig into that area deeper.
Ironically, modern day Israel is arguably the least violent in its foundation (recently at least - the original Ancient Israel was of course founded through conquest like everyone else, at least if you accept Old testament timeline as valid history). Modern day Israel ex-occupied-territories was established peacefully as part of UN vote - although, you can quibble that it was preceded by some terrorist activity against British. And no, the subsequent wars against invading Arab states don't count - they were not part of "founding" a country, they were part of defending its existence after founding (same way, WWII doesn't count for Russia or France, even if it started 1 day after it was founded). However, Arab Palestinians are there (recognized state or not) are there as part of conquest - depending on your choice of how far back to look, Ottoman conquest of the area; or earlier Arab Muslim conquests, or indirectly earlier, Roman wars which destroyed Ancient Israel and resulted in Jews being kicked out so Arabs could move in. This is Levant, there's always some war, conquest or fighting going on just like in Mesopotamia... prime real estate, baby).
The big exceptions I can think of, MAYBE, and mostly through my lack of knowledge of their less recent history, are Japan (but, see Ainu), Finland, Ethiopia (if you don't count defensive war against Italy), New Zealand (Maori were not conquered iirc).
Comments on the questions mentioned Iceland and Easter Island.