Is it really historically common for heads of state to be alone hammering out key negotiating points of a treaty? Given the notoriously rocky nature of diplomacy, gigantic stakes and extremely complex layers of interests usually involved, it seems like a lot for one-on-one discussions. Especially when there are so many ambassadors, diplomats, interpreters, secretaries, and other failsafes in place to avoid misunderstandings.
I'm wondering about the 20th century in particular, but if there's a general trend in the nature of how treaties are made I'd be very interested.
While not a head of state Henry Kissinger is an excellent example of private diplomacy in action.
It's through his private diplomacy that an opening to China was made in 1969 that swept away years of hostility from the Korean War and American support for the nationalist government in Taiwan.
He also attributes private diplomacy to achieving detente which led to a period of relaxed relations between the US and USSR over the 70s. The result of detente were arms limitations treaties such as SALT 1 and 2, the Paris Peace Accords and the Helsinki Treaty.
The way in which Kissinger managed to achieve so many treaty successes included bypassing much of the fail safes which you mentioned. This obviously ruffled a lot of feathers from Congress to the White House to the Pentagon, ultimately Kissinger's dictatorial hold on American diplomacy would be his own political demise as he became associated too closely with what many believed was a accommodation with communism.
Edits: I was writing this walking back from the station so I had to come back and correct a lot of grammar.
Sources Henry Kissinger - Diplomacy Henry Kissinger - The White House Years John Gaddis - Strategies of Containment Del Pero - Henry Kissinger: The Eccentric Realist
This sort of negotiation was more prevalent during the past than it is today, as one would expect given that the modern period has seen a the generation, and then professionalization, of the political bureaucracy. In modern times, it is somewhat difficult to actually classify any set of negotiations as personal diplomacy even when the appearance is there, as there is a large amount of machinery at work in various government evaluating any number of policy positions.
From our recent history, I would add to /u/Merrehmun's excellent post which brings up Kissinger, with an example where personal diplomacy was attempted but ultimately unsuccessful. At Reykjavik in 1986 Reagan was in summit with Gorbachev. At one point in the negotiations Gorbachev suggested coming together on a 10-year plan for complete nuclear disarmament, however, for a variety of reasons the deal fell through. No doubt the State Department and Soviet Foreign Ministry had prepared the ground for the potential paths this negotiation could take, but most accounts of the meeting make it clear that this was an actual negotiation between two heads of state.
If we go back into history this type of diplomacy seemingly becomes more common (although I hesitate to make a normative claim). Some of the more famous cases of personal diplomacy come from Napoleon (who else). While the legendary minister Talleyrand had major impact on French diplomacy during the period, Napoleon was often the primary 'negotiator' with heads of state. Probably the most famous example being the Treaty of Tilsit. This example is probably an excellent prototype to understand the power personal diplomacy has had in our history. During the negotiations Napoleon was able to extract such concessions from Tsar Alexander I that many speculated that Alexander was intimidated and, to use a modern phrase, star-struck by the Emperor.
Japan around the turn of the 19th century was involved in the creation of a number of treaties, specifically two treaties resolving wars and another renegotiating treaties that subjugated their position to Western powers.
With the exception of the treaty ending the first Sino-Japanese War, these treaties were signed and negotiated by the foreign ministers of the Japanese state. The Treaty of Shimonoseki ending the First Sino-Japanese War was negotiated by Itō Hirobumi, the prime minister of Japan. He was arguably given the position of negotiator because of his near unchallengeable status in the government of Japan. However, it is clear that he had a very strong understanding of Japan's position regarding the treaty negotiation, so it is understandable that he would receive this privilege. In fact, he and Mutsu Munemitsu, foreign minister at the time, both expected that Western foreign powers would intercede especially on the point of the Liaotung peninsula being ceded to Japan.
The revision of the unequal treaties, negotiated in 1894, and the Treaty of Portsmouth (1905) were both negotiated by foreign ministers. Mutsu Munemitsu negotiated the abolition of consular jurisdiction, most favored nation status, and unfavorable tariffs to Japan. The "unequal treaties" were exceptional in that Japan had treaties with over a dozen nations all in the same vein. So, revision was not so easy as revising one treaty with Japan and a dozen or so nations. Japan initially negotiated a revision with Great Britain, after which time the several other nations holding unequal treaties with Japan followed suit to revise.
The Treaty of Portsmouth was negotiated largely between Komura Jutarō, the Japanese foreign minister, and Sergei Witte, a Russian plenipotentary given the power to sign treaties under the Russian empire's name. That being said, even a glance at the wiki page shows more than two people negotiating at Portsmouth. And, this conference was of course set up Roosevelt, the U.S. head of state at the time.
So, to answer your question, Japan at this time may not fit the "House of Cards" picture. The negotiation of the Treaty of Shimonoseki in 1895, however, fits this ideal closest because it was essentially negotiated between Itō Hirobumi and Li Hung Chang. The other two treaties I mentioned do not fit so nicely into that sort of platform.
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