The first key point you are missing is 1895, in which the first Sino-Japanese war took place. Up until this point Korea had been under Qing domination. The Japanese victory here gave Japan its first "overseas territory" in Taiwan, which immediately revolted and then was militarily put under Japanese control. Its important to understand that while Modern Korea really isn't superbly fond of Japan, there were factions within Korea in the 1880s and 90s that were actually in favor of Japanese involvement over that of Qing, and even managed to temporarily overthrow the pro-qing government in 1884 which was put down by Yuan Shikai and led to a mutual pact that had the effect of undermining Chinese control of Korean politics.

However the victory in the first Sino-Japanese war basically gave Japan unilateral domination over the Korean peninsula and ended Chinese power in the region, the only serious threats to Japanese control were the Russians and potentially the Americans.

The next key point would be the Russo-Japanese War in 1905, in which the Japanese were able to get the european powers to not intervene over potential conflicts on the Korean peninsula. Basically after this Japan sent a series of three successively more dominating treaties, ending with the annexation of Korea in 1910.

There was a history of political machinations both in and outside Korea by Japan determining it's fate.

The first political contact between Korea and modernized Japan happened in 1876 when the Japanese forced Korea to open to Japanese trade in that year by borrowing a page from commodore Perry. This treaty also disengaged Japan from the traditional model of Sino centric relations has Japan renounced its official status as a tributary state to China.

Korea traditionally had maintained an alliance/client patron relationship with China (Imjin War of 1592-1598). But with the decisive defeat of China to the Japanese during the 1st Sino-Japanese war (1894-95) the Sinocentric model of international relations in East Asia collapsed which left Korea exposed while the Japanese occupied Taiwan and other bits of strategic territory of the Chinese coastline. During this time Empress Myeongseong who was a strong Korean figure who held similar view towards modernization as the "self strengthening" movement in China was assassinated by agent with connections to the Japanese litigation in Seoul.

Following this China was seen as in a similar situation as the Ottomans in Europe as crumbling giant that was easy prey for stronger nations which brought in Russian attention. The Russian were at this time tying together there Pacific territories to their European heartland with projects like the trans-Siberian parts of which went through China to reach Vladivostok. Viewing the Japanese as a threat to this strategic artery while also eying more Chinese territory as were the Japanese who also viewed the Russians as a threat. This culminated in the Russo-Japanese war (1904-05) which was fought by both sides on both Korean and Chinese territory and saw the Japanese decisively defeat the Russians which left them as the only major power based in east Asia with Japan occupying more the Liaodong peninsula.

The war was followed by the Japan-Korea treaty of 1905 which made Korea a protectorate of Japan which was followed by the treaty of 1907 which made the Japanese resident general effectively the chief executive as the Koreans couldn't pass laws without his approval, he could replace important staff both in the civil service and military and finally had to follow his directive as law. This followed the 1910 treaty in which Japan formally annexed Korea.

Now to finally get around to answering your question you can see why the formal military didn't put up a fight as it was both outclassed but had also been purged from the administration. However many Korean administrators formed "Righteous Armies" (which were named from similar formations formed during the Imjin War) which were ad hoc formations made up of local militias, former military and peasants. So while there was armed resistance it wasn't the formal military and most of it was put down by the start of World War One as the Righteous Armies had not industrial base from which to prosecute a conventional war that could have overthrown the Japanese.

An old Korean saying is that "A shrimp's back breaks in a fight among whales".

There are 3 books that you should read on this topic. Peter Duus: The Abacus and the Sword; Alexis Dudden: Japan's Colonization of Korea; and Jun Uchida: Brokers of Empire.

These books respectively describe: The mutual interaction of settlers and government actors working with different goals in mind that eventually forced annexation from a Japnese perspective; the calculated Japanese use of international law to eliminate Korean sovereignty and thereby enable annexation; and the actions of Japanese settlers in securing Japanese control of Korea.

Overall, your question is slightly off base - Japanese control was already nearly complete at the time of annexation. You need to understand that Korea was a Japanese protectorate after 1905 - annexation wasn't a brand new thing, but rather a comparatively small escalation of the long standing Japanese influence in the peninsula, marked by continuous and gradual escalation. Moreover, by that time, the international community had acknowledged Japan's legal right to speak on behalf of Korea. (that doesn't make it right, but imperialism was quite legal in the early 20th century)

Ever since the 1876 Treaty of Kanghwa, Japan had at least an advisory role in Korean government. The full history is decidedly more complex, but suffice it to say that between 1876 and 1905, the Russians, the Chinese, and the Japanese were all competing for influence in Korea, allying with various factions in the Korean government, and generally causing a ruckus while trying to secure concessions there. The Sino-Japanese and Russo-Japanese wars decided that competition firmly in favor of the Japanese, with Japan winning the internationally recognized right to administer Korea as a protectorate after the Russo-Japanese war. Meanwhile, Japanese commercial interests, which military and political actions had enabled, had secured important economic concessions in the peninsula and established markets there, which simultaneously demanded and enabled the expansion of Japanese political and military influence. The initial protectorate plan had been to encourage a collaborationist government in Korea, but that didn't work (who was really going to collaborate?), so Japan took the continued presence of "righteous armies" opposing Japanese rule and the assassination of Itō Hirobumi as an excuse to completely annex the Korean Peninsula.

So yes, 1910 might look significant, but considering the events of 1895 and 1905 and the extensions of Japanese power in between, annexation was more of a straw that broke the camel's back than a radical change.