The count of disappearing countries stands as this:
1. Neverwonagame3's Xenshin (Korean Peninsula, Asia), year 2500 BC, finished off by Koryo after a long war.
2. Oregon (Western Seaboard, USA, America), year 2500 BC, annexed by California after a war.
3. Mexico (Central America), year 2500 BC, annexed pacifically by Florida.
4. Sioux (Central-Eastern USA, America), year 2100 BC, annexed by California
5. Stormbringer's Carthage (North Africa and Sardinia, Europe), year 2100 BC, defeated by the Tartessian and Milanese armies and fleet after trying to run away to Sicilia.
6. Fëanor's Dani Tribe (Denmark and Scandinavia, Northern Europe), year 2100 BC, defeated by the Channel Union after a long, bloody war.
7. qoou's Sicilia (Southern Italy, Europe), year 2100 BC, annexed by Tartessos and Milan after civil revolts explode and divide the country.
8. Churchill 25's California (Eastern USA, America), year 2000 BC, beaten by the Three Kings Alliance.
9. Flavius Aetius' Kingdom of Milan (Northern Italy, Europe), year 1800 BC, culture-flipped by Tartessos and annexed.
10. D'Artagnan59's Bengal (India, Asia), year 1800 BC, annexed by Betal-Dare and the Empire of the Rising Sun.
It ain't bad, 10 nations that have been defeated, so far! If anyone didn't like being reminded of his/her defeats, I am quite sorry if I offended you, I just wanted to make a message Re: this.
EDIT: Nations that have annexed and/or destroyed other nations: Tartessos (3 nations: Carthage, Sicilia, Milan), Kingdom of Milan (2 nations: Carthage and Sicilia), California (2 nations: Oregon and Sioux), Florida (2 nations: Mexico and California), Channel Union (1 nation: Dani Tribe), Massachussets (1 nation: California), Chicago (1 nation: California), Betal-Dare (1 nation: Bengal) and the Empire of the Rising Sun (1 nation: Bengal). I am only counting here those nations which actually finished off other nation. If we had to put the other nations that fought against them, it would include Sicilia and the Hellenic Republic (1 nation: Carthage) and Mexico and Oregon (1 nation: California).
EDIT 2: I am a nation-killer!