The downloadable treasure trove at the National Assembly Library website

Update:

If you click on one of the items in the search results and open a new page, and then scroll down, there is an index of what the file contains. As well, once on that page, right click on the 다운로드 button and save the link address to get a permalink for that page. For example, the page for the file which contains the cable I transcribed below is here.

As well, old maps of Seoul - and elsewhere - dating back to 1900 can be found here. To download a map, click the 다운로드 button at the bottom, then scroll back up to the top to see the window (or that's how it works in Chrome, at least) and then next to '사용목적'  choose your purpose for downloading (the first option is 연구, or 'research', and then type out a brief reason (or at least enter something in the box), and then click the 확인 button at the bottom of the window.

Original Post:

Readers might not be aware of the treasure trove of information at the National Assembly Library's website. For example, they have the drafts of bills, bill revisions, and minutes of parliamentary discussions all available for download. They also have a good deal of declassified US State Department cables from the 1950s to about 1973 (as well as similar documents on microfilm going back to the Joseon period and colonial period).

To find the latter, search for 'internal affairs of Korea,' and then among the results look for those with a 원문보기 and 다운로드 button. Click the latter and the first time you do it should prompt a download of their viewer program; once you have that you can download a great deal of material, including Status of Forces Agreement Joint Committee meeting minutes.

Another source of similar information is db.history.go.kr, where a number of similar US diplomatic cables can be found. Another interesting find there is most of History of the United States Army Forces in Korea, the US military's official history of the US military government of 1945-48. There's also the National Library of Korea, which has a digital archive, such as this one featuring thousands of Korean textbooks.

As for the material at the National Assembly Library website, the 'internal affairs' material can be frustrating due to it often being not in any order, but some fascinating material can be found for those who persist. I'll try to post some of the more interesting things I find. To start, here is a report from May 26, 1971.

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Department of State Briefing Memorandum 7209350
May 26, 1971
Top Secret – Nodis

To: The Acting Secretary
From INR – Ray S. Cline
US-ROK Explosion: Ambassador Habib’s worries

I offer the following observations on the problem raised in Ambassador Habib's message. (SEOUL 2869, May 19.)

1. US-ROK relations have never been untroubled. In fact, strong disagreement, often public, and mutual suspicion on matters of major concern to both sides have been familiar features of US-ROK relations for the past 20 years. Syngman Rhee violently opposed the Korean Armistice Agreement and wanted continued fighting until Korea was unified. We feared and opposed his subsequent loudly proclaimed intention to march north. Our Mission in Seoul initially opposed General Pak‘s coup in 1960 and exerted the strongest pressure in 1963 to bring about Pak's reluctant agreement to hold elections. The contrast between our response to the seizure of the Pueblo, and to the Blue House raid a short time before, infuriated Pak.

2. Ambassador Habib‘s estimate that there is at present a greater degree of ROK suspicion and uncertainty concerning US intentions may well be true. We can neither deny nor affirm this. But we submit that the situation he describes is not created by our lack of consultation with Pak; rather, it is the result of Pak’s great unhappiness with our policies. Basically, he is worried that we are getting out and he doesn't like it:

-- For Pak, the most direct evidence of our intentions was the withdrawal of one US division from Korea (announced without previous consultation). The withdrawal excited the most extreme reaction, including the threat of the Government to resign.
-- Pak is unhappy over our Vietnam policy. He believes that we must now allow the communists to win and is afraid that we are running away. He fears we will repeat this performance in Korea. (He also intensely dislikes the way we treat the ROK role in Vietnam as essentially mercenary.)
-- He was stunned by the President's China trip and sees the improvement in US relations with China as a cover-up for our withdrawal from Asia.

Pak sees his basic worry confirmed in Congressional attitudes toward Vietnam and military assistance. There have also been other pin-pricks -- our privately expressed concern over the December emergency decrees and our public disagreement with the ROKG over the imminence of a North Korean attack.

3. While Pak's unhappiness over our policies and lack of meaningful consultation is cause for concern, we do not believe there is much danger he will vent his unhappiness in ways seriously detrimental to our interests. (We categorically rule out any effort to involve us in North-South hostilities.)

a. His major leverage on the US comes from the presence of ROK troops in Vietnam. Their dispatch to Vietnam has obtained important political and economic benefits for the ROK. These troops are, for the present, perhaps the major determinant of US policy in Korea. But ROK troops are clearly moving out, the inevitable result of the departure of our own combat troops from Vietnam.
We want two things from the R0K in this regard: to expand the operational area of ROK troops in Vietnam and to keep them there as long as possible, and at least through FY 1912. The first, a direct Presidential appeal, has already been clearly and, not surprisingly, rejected by Pak and coupled with a demand for increased assistance for their forces before he agrees to the latter. This direct Presidential request, however, has been instrumental in obtaining our second desire, since it has put pressure on Pak not to turn down for a second time a Presidential request. The ROK has apparently now given us virtually a commitment on their troops remaining through CY 1972. Knowing the importance President Nixon attaches to the ROK not pulling out at this time (as well as his own concern for the outcome in Vietnam), Pak is not likely to want to jeopardize US support for Korea in a game of chicken.

b. The ROK is aware of our interest in reducing North-South tensions. Conceivably out of pique toward us the ROK might seek to stem further movement in this direction. We believe this is very unlikely: it undercuts Pak's domestic political position without insuring that he will gain anything vis-a-vis the US. Rather, Pak has adopted the opposite tactic, i.e., the Yi Hu Rak mission for which we already have congratulated the ROK. We would not be surprised if our dealings on myriad smaller issues becomes more difficult.

4. Certainly we should allay ROK suspicions as to our policies to the extent we can, particularly on direct US-ROK issues. We doubt, however that prior consultations on China and Vietnam are possible or even desirable. Our policies In these areas may not please Pak and to consult with him could lead to even more bitter recriminations. But this does not relieve us of trying to put our actions in these areas in perspective for the ROK. We can probably do much better in this regard.

5. However, it should also be recognized that recriminations and psychotic postures are a major negotiating tool with the ROK, one which they have used with considerable efficacy on us. For Pak to vent his violent displeasure may not result in obtaining his wishes as to the matter at hand; but the intensity of his response makes the US think twice before doing anything that might affect Korea.

6. Nevertheless, over the longer term US interests would clearly suffer should a deepening ROK distrust of the US policies and a growing feeling that they were being ignored by the US lead to a sharp and visible deterioration in ROK-US relations. Such a change could damage internal stability in the ROK and cause Pyongyang to reassess Seoul's overall strength. This in turn could lead either or both Korean states to return to violent tactics of North-South competition with a consequent increase in tensions among the major power allies of the two Koreas.

7. Thus, we have no real quarrel with Ambassador Habib‘s overall pitch for taking the ROK more into our confidence, but we do not view the present situation with alarm.

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It says above that in regard to "Ambassador Habib‘s estimate that there is at present a greater degree of ROK suspicion and uncertainty concerning US intentions" that "the situation he describes is not created by our lack of consultation with Pak; rather, it is the result of Pak’s great unhappiness with our policies." It's worth noting that Katherine Moon argued the opposite in her book Sex Among Allies: Military Prostitution in U.S.-Korea Relations. There she pointed out that the US committed a number of gaffes in the way it withdrew 20,000 troops from Korea in 1970 (as part of the Nixon Doctrine) that frustrated and upset those in the South Korean government and military to no end. Obviously the author of the above report lacked understanding of how form is often just as important as content in Korea - sometimes even more so.