Some history facts;
 
Three Kingdoms of Korea:  
Korean peninsula was divided among three kingdoms, Baekje, Goguryeo and Silla. At the end, Silla conquers and unite with other kingdoms with help of China
 
Goryeo:  
After collapse of united Silla and chaos, Goryeo was founded, trying to imitate the old Goguryeo and its military power, hence the similarity in names.
 
Joseon:  
Goryeo was revolted at the end and new kingdom, Joseon, was made. Unlike Goryeo, Joseon focused more on literature and Confucianism than military power.
 
The Great Korean Empire:  
By the end of Joseon, Joseon king attempted to modernise and accept western culture, as far as renaming the country as the Great Korean Empire. However, by this time, Korean government was corrupt and rotten to its core, which made itself weak and vulnerable to foreign attacks
 
Korea under Japanese rule:  
After Japan has successfully won the First Sino-Japanese War and dealing with corrupt Korean government, Japan was able to annex and colonise Korea, performing atrocities against Korean population
 
North and South Korea:  
This is Korea we have today. After Independence from Japan after their loss in WWII, Two halves of Korea, North and South, went under coccupation of Soviet Union and USA respectively. Then North Korea invaded South resulting in Korean War, which solidified the division after the war was stopped under Armistice.
 
Korean Unification flag:  
Technically, this is ponification of flag not a country since North and South are still divided in present days, but the flag represents unified Korea and is used when North and South are in sports event together