It wasn't a country before the Korean War, at least not for about 40-55 years before.

Korea was annexed by Japan in 1910, becoming part of the Japanese empire until 1945. Prior to annexation, it was a protectorate from 1905 meaning that it had no sovereign authority in international affairs. Prior to even this, the capital (Seoul) had been occupied by Japanese soldiers since the end of the First Sino-Japanese War in 1895.

With that in mind, Korea under Japanese occupation was:

Economically

Growing. The Japanese occupation was good for Korea in the economic field. The first railroad in Korea was the Gyeongin line running from Noryangjin Station, just south of the Han River and the capital, Seoul, to Jemulpo Station, Incheon. It was originally to be built by an American company, but they sold the rights to a Japanese company who finished the line. A note on the names - the train lines are generally named for their location. "Gyeong" means capital and "In" is Incheon. During the Japanese colonial era, the Gyeongbu(san) line was built, as was the Honam line (Jeolla region, southwest), and the Gyeongui line, connecting Seoul to Pyongyang and Sinuiju on the Chinese border. The Japanese build heavy industrial factories in North Korea (North Korea is a poor place for farming, but is decently well off with natural resources - ores and minerals) and brought newer farming techniques to South Korea (much better for farming, but poor in natural resources). Large fertilizer factories in the north supplied the farms in the south and in Japan. Economically, things were pretty good.

The downside to this is that the Japanese themselves had taken over much of the capital resources - factories, farms, businesses, etc - and they reaped the lions share of the rewards. Still, a rising tide lifts all ships.

Culture

The Japanese waged a culture war against Korea. They brought in Japanese education (which was good) but which focused on Korea as a part of the Japanese empire. The Japanese goal was assimilation and that meant crushing Korean culture. In the early part of colonization, Korean publications were banned. Shinto shrines were erected in every town with a particularly large one built on the side of Namsan Mountain in Seoul. As one of the four peaks of the capital, this was particularly offensive to Koreans. In the 20s, Korean publications were allowed again, but they were heavily supervised. At the end of the 30s, the Japanese doubled down on the culture wars and required anyone doing business with the government to use Japanese names. Many older Koreans today can tell you what their Japanese name was (and they'll say it with a sneer and then hock a big loogie onto the street!).

Pre-colonization

Prior to Japanese colonization, Korea was "a shrimp stuck between two whales." Korea had always been the bridge between Japan and China and had frequently been attacked and invaded when those two countries went to war. Historically, Korea had benefited from its place in the middle, serving as a trading point, but they were still the smaller country. Korean culture has always borrowed from Chinese culture and Joseon (the kingdom from 1392-1910) had been a tributary state to China until the late 1800's.

Korean War

Syngman Rhee was as fanatical about reunification as Kim Il-sung was, so much so that the US refused to outfit his military the way he wanted for fear that he would start a war. This is a huge part of why South Korea was so unprepared for the Korean War. Did Korea expect the US to use nuclear weapons? That's kind of a red herring. Syngman Rhee wanted the US to help him crush Kim Il-sung and reunify Korea under his regime. If it took nuclear weapons, I'm sure he would have been behind it 100%, but at the outbreak of the Korean War, there were only a few hundred US soldiers in Korea. They were advisors to the ROK military and when fighting broke out, some of them were captured immediately. The more immediate concern was not nuclear weapons, but simply military aid and support. US forces in Japan were woefully unprepared having gotten fat and lazy doing occupation duty and Truman had been cutting the military down so that the US was no longer able to fight the kind of war that Korea was to become and still maintain a line against the Soviets in Europe. It took from June until September until the US was able to begin a counter attack, and even then it was only with the help of the UN forces, namely the UK/Commonwealth countries.