Since normalizing their diplomatic ties half a century ago, South Korea and Japan have made substantial progress in the economic, social and cultural domains, with trade volume nearly 400 times what it was.
But their deepening partnership has been overshadowed by historical and territorial conflicts that have escalated in recent years amid their lack of high-level dialogue, political distrust and rising domestic nationalism.
Seoul's Prime Minister Yun Byung-se's visit to Tokyo on Sunday has raised hopes for a much-needed momentum to mend fences and look beyond the historical disputes involving Japan's 1910-45 colonisation of the Korean Peninsula.
But the two neighbours' long-festering conflicts seem to be so deep that some even cast bilateral ties as a "zero-sum game," in which one side's gain means the other side's loss, or even a "minus-sum game" where both sides lose out due to their insistence on pursuing their own interests. Yun rejected this view.
Noting the difficulty erasing the "collective memories" of the past, experts say the leaders of the countries should employ a more "active and creative" diplomacy to minimise their conflicts and maximise co-operation in handling shared challenges such as regional security and demographic changes.
"There are many areas where the two countries can see a convergence of their strategic interests, including North Korea's military threats, protection of global commons such as maritime transport routes and maintenance of the regional status quo," said Nam Chang-hee, an international politics professor at Inha University.
"They should strive to frequently hold dialogue to better understand each other, find common ground, and expand their co-operation, which will help ease their historical tensions. That is what diplomacy is all about."
With Washington's push to bring its key allies of South Korea and Japan together in a Cold War-era campaign against the former Soviet Union, the two sides reached a landmark deal to normalise their relations on June 22, 1965 - after some 14 years of grueling negotiations.
The normalisation deal was struck amid intense opposition in Korea, where memories of Japan's colonial atrocities were still potent.
A day after the signing of the deal, then-President Park Chung-hee, the late father of the incumbent president, told his nation, "It is wise to grab the hands of even a onetime foe, like Japan, if it is needed to promote the well-being of our people for today and tomorrow."
Though it failed to settle historical issues, the outcome of the deal reflected the delicate balance between the two sides' interests.
On the back of the deal, Korea, then faltering in the aftermath of the 1950-53 Korean War, laid the foundation for its economic rise, while Japan, criticised for its wartime atrocities and aggressions, was given a chance to restore regional trust.
"The deal was, at the time, seen as mutually beneficial. From a pragmatic standpoint, Korean officials viewed the deal as a way to facilitate their country's industrialization to move beyond the ashes of the war," said Kim Soung-chul, senior fellow at the local think tank Sejong Institute.
"Though historical issues were pushed aside and left unresolved (in the process of the negotiations), the deal was a turning point in the checkered history of the bilateral relations."
Under the "Agreement on the Settlement of Problems Concerning Property and Claims, and on Economic Cooperation between Japan and Republic of Korea," Tokyo pledged to offer to Seoul US$300 million (S$400 million) in free grants, US$200 million in long-term and low-interest loans and US$300 million in commercial loans.
The financial assistance to Seoul was offered not in the form of reparations for damages from Japan's brutal colonisation of the peninsula, but as "economic co-operation" and the settlement of property claims.
The deal, thus, did not include any apology or words of repentance on the part of Japan for the occupation of its territory, despite Korea's repeated calls during the negotiations.
Experts say this has left the deal "incomplete and unfinished," although they recognise the agreement was an "inevitable compromise" for a poor Korea whose per-capital gross domestic product was merely around US$100 a year at the time.
"As Korea's top priority at the time is to pluck its people out of poverty and push for economic development, it needed technological and financial support from Japan ― a reason why historical issues were put on the back burner," said Kim Ho-sup, diplomacy professor at Chung-Ang University.
"Now, the historical issues are things to be complemented to 'fully normalise' the bilateral ties ― a tough task, which was left behind for the next governments."
The different interpretations of the normalisation pact have also been a source of tension between the two countries.
But Seoul claims the agreement did not settle all issues stemming from Japan's colonial rule, and that it did not deal with violations of individual human rights such as Japan's wartime sexual enslavement of Korean women.
Cooperation eclipsed by conflicts
The considerable progress in the two sides' co-operation in various areas has been eclipsed by conflicts over Japan's distortions of history in school textbooks, repeated claims to the Dokdo islets under Korea's control and its refusal to explicitly apologise for its wartime sexual slavery.
But experts say that in terms of bilateral co-operation, the partnership has fared well, and that the two countries should strive to prevent political and historical tensions from damaging cooperative relations.
According to recent data from the Korea International Trade Association, the trade volume between Korea and Japan, which was some US$220 million in 1965, rose to around US$86 billion last year.
Japan is now Korea's third-largest trading partner after China and the US Korea is also the third-largest trading partner for Japan.
"Despite political conflicts between the two nations, the quality of their economic co-operation has improved, though the quality of it has somewhat decreased as Korea's economic exchanges with a rising China have increased," said Lee Jung-hwan, Japan expert at Kookmin University.
"Still, Korean firms participating in the 'global value chains' rely much on Japan for securing their component parts and production facilities to manufacture their final products," he added, referring to international networks to optimize manufacturers' production processes.
Lee pointed out that amid worsening historical feuds, people in both countries seemed to take their economic co-operation for granted "just as they take air and water for granted."
However, historical tensions appear to have negatively influenced bilateral trade to a certain extent. Two-way trade volume, which rose to a record high of US$108 billion in 2011, has fallen over the last four years.
Over the course of 50 years, people-to-people exchanges have also dramatically risen. The annual number of Korean and Japanese nationals who travelled between the two countries has soared to more than 5 million from merely around 10,000, including around 5,100 Japanese visitors, in 1965.
Experts say the increase in people-to-people exchanges is attributable largely to the two sides' active economic exchanges, open-door policies toward each other and the pan-Asian popularity of Korean pop culture.
Relations deadlocked in history trap
Since Prime Minister Shinzo Abe began his second term in December 2012, his rhetoric and action, which critics say reveal his "revisionist" view of history, have worsened diplomatic friction with South Korea.
From his remarks in April 2013 on the "ill-defined" definition of an invasion to his visit in December that year to the Yasukuni Shrine honoring the country's war dead, including 14 Class-A war criminals, Abe's nationalist moves have called into question his political will to mend ties with Korea.
Last June, Tokyo carried out a controversial "review" of the 1993 Kono Statement, in which then-Chief Cabinet Secretary Yohei Kono apologised to the victims of sexual slavery. After the review, Tokyo suggested the apology was the product of a political compromise between Seoul and Tokyo, challenging its credibility.
The Abe government's decision to send a senior official to the Feb. 22 "Takeshima Day" celebration to assert its sovereignty claim to Dokdo has further exacerbated the relations. The Japanese prefecture of Shimane has observed the day for Takeshima, the Japanese name for Dokdo.
Tokyo has also claimed sovereignty to Dokdo in its diplomatic and defence white papers.
The bilateral conflict over Dokdo peaked in August 2012 when former President Lee Myung-bak visited the islets. After the visit, Tokyo made a proposal to Seoul to bring their spat over Dokdo to the International Court of Justice. Seoul rejected it, arguing there is "no dispute whatsoever" over Dokdo.
Adding fuel to the historical spats is Japan's bid to put 23 Meiji Industrial Revolution sites, including those related to Koreans' colonial-era forced labour, on the UNESCO World Heritage list.
Seoul has demanded Tokyo exclude seven of the sites, where nearly 57,900 Koreans were forced to work during the colonial era ― from the application, or to clearly present the facts about forced labour when they are designated as heritage sites.
Above all, the thorniest historical issue is that of Japan's wartime sexual enslavement of Korean women, euphemistically called "comfort women."
Seoul has long called on Tokyo to take "legal" responsibility for the victims and officially apologise to them, stressing that the issue is urgent as there are only 50 surviving victims with many having died. Tokyo claims the issue was already settled under a 1965 normalisation agreement.
Although their differences over history seemed irreconcilable, all the bilateral efforts to address them have not gone down the drain.
In 1998, then Japanese Prime Minister Keizo Obuchi expressed "deep remorse and a heartfelt apology" to Korea for Japan's colonial rule in a joint statement issued at the close of his summit with then Korean President Kim Dae-jung.
Seoul also secured the 1993 Kono Statement and 1995 Murayama Statement that expressed Tokyo's apology for its wartime sexual enslavement of Korean women and its colonisation of the peninsula, respectively.
Experts say that Seoul and Tokyo should enhance high-level communication to help them better understand each other, remove misunderstandings and restore bilateral trust.
Noting that historical issues can hardly be completely resolved, they also said the two sides should find ways to manage them well, and prevent them from escalating and hurting mutually beneficial co-operation in trade, security and other areas.
On the part of national leaders, they should not ride the wave of growing nationalism to improve their political positions, the experts added.
"Both countries need to focus on the future and their shared democratic values, respect for rule of law, emphasis on free market economics and the many things they share," T. J. Pempel, political science professor at the University of California, Berkeley.
"Both sets of leaders should agree not to talk about 'the history issue' for a decade or more."
Nam suggested that Seoul and Tokyo strive to find shared challenges for which they should step up bilateral co-operation.
The challenges include their slowing economies, demographic issues stemming from their aging populations, North Korea's military threats and an ongoing shift in the regional balance of power caused by the rise of China.
Some experts also noted the need to increase exchanges among young people of the two countries who are largely free from the influences of the colonial past and can better understand one another through various high-tech tools, including social networking services.