Okay, so a detailed answer now.

Although the Empire of Korea was only officially annexed to Japan in 1910, the Eulsa Treaty of 1905 effectively made Korea a Japanese protectorate (this was only a few months after the Russian defeat in the Russo-Japanese War). This resulted in armed revolts, called Uibyeong or "Righteous Armies" in English. By June 1906, there were revolts in over 60 different locations throughout the peninsula. The Emperor of Korea, Gojong, may have secretly encouraged revolts.

The major incidents of the 1905-1906 revolts were

!. The Siege of Hongju Fortress, when 1,000 Uibyeong seized the fortress of Hongju in southwestern Korea. The Siege lasted for more than 10 days, but eventually the leader was captured by the Japanese.

2. The various skirmishes between the Japanese and Shin Dol-Seok's guerrilla forces in the eastern coastline.

3. Choe Ik-hyeon, an influential Confucian scholar, also raised men, but when Korean troops were sent to suppress his revolt Choe refused to fight his emperor's own troops and the revolt ended bloodlessly.

Meanwhile, many Koreans disagreed with armed revolts, and believed they could strengthen Korea through peaceful methods. Hundreds of schools were newly built, and various newspapers were founded. There were movements led by the public, like a movement to raise money to pay off Korea's national debt, and a movement to stop the Japanese from obtaining rights to develop (largely) uninhabited regions of Korea.

Emperor Gojong was forced to abdicate in 1907, causing more Righteous Army revolts. More importantly, the Korean Army was dissolved in 1907. The Korean troops in Seoul were mostly killed after a battle with Japanese soldiers, but many soldiers in the provinces deserted. About 5,000 former Korean soldiers are estimated to have joined Righteous Armies. In 1907, 10,000 Uibyeong soldiers (about 0.07% of the Korean population) decided to attack Seoul, but the father of the commander died, and the army broke apart back into small groups. By 1912, Japanese campaigns had wiped out most Uibyeong in the south (especially in the southwest) and the remnants fled to Manchuria and the Russian Far East.

Another incident occurred in 1909, when a Pan-Asianist named An Jung-Geun assassinated Ito Hirobumi, Prime Minister of Japan.

Korea was officially annexed by the Empire of Japan in 1910, but not much really happened except some Koreans committing suicide (by 1910, most Uibyeong were already dead or in Manchuria/Russian Far East).

Not much happened in the 1910s (there was a strong military presence in Korea), while the world was fighting WWI. But by 1919, Koreans had heard of the various newly independent states in Europe (a result of the disintegration of Austro-Hungary and the "right of nations to self-determination" following WWI) and decided to proclaim Korean independence. In March 1st, 1919, 33 representatives from the three main religions of Korea at the time (Cheondoism, Christianity which was mostly Catholicism, and Buddhism) were to read out the proclamation of Korean independence to the public at Pagoda Park in modern Seoul, but fearing an armed revolt by the populace, they decided to read the proclamation separately from the public.

However, the public in Pagoda Park declared independence on its own without the representatives, and the March 1st Movement began. The Movement was originally mostly peaceful protesing, but gradually it became somewhat of an armed conflict with attacks on government offices and "battles" against Japanese troops. About 7,500, mostly Koreans, died in the protests.

But when in June 28, Korean independence was not mentioned in the Treaty of Versailles (the Koreans had expected Woodrow Wilson to oppose Japanese rule over Korea, but that never happened), the March 1st Movement rapidly declined and by September it was more or less dead.

As a result of the March 1st Movement a Provisional Government of Korea (PGK) was founded in Shanghai in September 1919. Originally, it actually published a newspaper and funded Korean independence activists in Korea. That is to say, it actually did stuff.

Within PGK there was a conflict between people who wanted an armed conflict and people who wanted peaceful independence. This conflict grew increasingly worse, and from 1923 onwards it was nearly dead. Kim Gu managed to keep it barely alive, orchestrating the Sakuradamon Incident (a failed attempt to assassinate Hirohito, Emperor of Japan) and the Shanghai Bombing (a successful bombing that killed a number of Japanese military officials, as well as a presumably innocent doctor).

On the other hand, there were armed revolts in southern Manchuria. Guerrilla groups often raided northeastern Korea, and there were occasional large-scale battles like the Battle of Fengwudong in June 1920 and the Battle of Qingshanli in October 1920. However, even in these rare large battles Japanese casualties may have never exceeded 1,000 (while there were 15,000 Japanese troops in Manchuria by 1920).

By the 1930s, most Korean guerrillas in Manchuria were Communists, including Kim Il-Sung. The 5,000 Communist guerrillas allied with the Chinese and fought the Japanese in various skirmishes, including the Battle of Bochonbo in 1932, where a few dozen Japanese were killed. Of course, North Korea has turned the Bochonbo skirmish into the greatest battle that was ever fought. But by 1939, even the Communists were forced into the Russian Far East, where Kim Jung-Il was born.

Within Korea, in the 1920s Japan decided to be less harsh on its Korean subjects, and the Korean standard of life improved somewhat. There were some minor protesting throughout the Japanese rule, but the March 1st Movement was the only movement that was national. The Singanhoe, a coalition of Socialist and non-Socialist nationalists, briefly had thousands of members but was forcibly dissolved in 1931. By the late 1930s, most non-Communist Korean intellectuals had given up Korean independence and decided that assimilation into Japan would be the best way for Koreans to develop.

The Provisional Government of Korea continuously relocated as Japan advanced into China. By the 1940s, it was based on Chongqing. The PGK formed a "Korean Liberation Army" in 1941, which fought with the British in Burma against the Japanese. By 1945, the KLA had swelled to about 1,000, but then WW2 ended and Korea was suddenly independent.

Now, was the Korean independence movement a serious challenge? I disagree. The March 1st Movement could never have been successful, partly because the Japanese soldiers were far better equipped than Korean civilians, and partly because much of it was based upon the illusion of American support. The PGK was moribund for 23 years during its 26-year history. The various guerrilla groups in Manchuria could not win, either, as seen by the fact that they were all gradually forced into the USSR. Think modern Xinjiang: the Uyghur independence activists aren't a serious challenge to the Chinese government. Same thing.

I can't give a sourced, detail answer since I'm travelling (in Vienna right now, apologies in advance) but I can give you a few things off my head you can look up at your own leisure.

There were a whole host of protesting going on, the most famous being the the** 3.1 movement.** Other famous incidents such the shooting of Ito Hirobumi by Ahn Jung Gun and the "lunchbox bomb" incident (although it was a water bottle) by Yun Bong Gil were also violent acts against the occupation, but were not necessarily the actions of an organised resistance, but were in line with Kim Gu's policy of targeted assassinations.

There were major and minor resistance armies, mostly based in Manchuria or the Russian far east. Kim Jwa Jin is the only major military figure that comes to mind.

The was a Korean government in exile, in Shanghai, headed by Kim Gu and Rhee Seung Man at different points if I remember correctly.

Hope that gets you started! I'm aware there isn't too much information on the internet but I've seen english language books about the topic before!