Basically, every country with its neighbours.
This is true to a certain extent, although the opposite is also frequently true because endless wars eventually exhaust both states' capabilities.
Denmark and Sweden is with a fair distance the two nations that have declared war on each other the most times.
This is debatable because it depends on exactly how you define a "country" and a "war." There is no definitive answer.
This is fundamentally the answer.
The exceptions are actually more interesting than those areas following the trend.
Despite examples providing by Democracy101, East Asia is actually exceptional in having longer periods of peace between neighbors than almost any other region-
Maybe, but the comparisons are somewhat flawed because they're comparing entirely different time periods.
Japan and Korea, only three wars in 1,000 years
China and Vietnam, only 3 wars in 1,000 years.
China and Japan, only 3 wars in a 1,000 years.
Despite modern nationalistic antagonisms suggesting frequent war, these East Asian states fought each other less often than most countries in close proximity to each other. They just remember it harder.
According to this logic, China and Korea only fought 3-4 wars within a thousand years as well, but that's only after ignoring the 15+ conflicts for 1000+ years before then.
Specifically, Japan arguably wasn't fully consolidated until the 8th century or so, soon after the Taihō Code was enacted in 703. The Battle of Baekgang (which, as the name implies, was technically a "battle" that was merely one of many in the extensive conflicts between Goguryeo, Baekje, and Silla against the Tang) illustrated the severe unpreparedness of Japanese troops in a direct conflict with its neighbors. In particular, the battle was arguably more of a restoration movement led by Baekje aristocrats, while the Japanese side's severe lack of logistical knowledge eventually resulted in a catastrophic defeat despite Japanese ships outnumbering those of the Silla-Tang alliance by almost five to one. As a result, neither Japan nor its neighbors generally bothered to initiate war for more than a millennia, as pressing issues were generally resolved through tribute.
(Chinese invasions into Japan would also have had to require "conquering" Korea for logistical support, of which the latter never really occurred for numerous reasons.)
Goryeo under the Mongol invasions of Japan technically don't count either because the peninsula was under duress from the Yuan, not to mention that Korea covertly sent aid to Japan shortly before the invasions for geopolitical reasons. This only leaves the Imjin War, which in itself was an extremely devastating invasion because it was the first and
only time that the peninsula had been invaded from the south, resulting in lax preparations despite a largely capable army stationed in the north against the Jurchen at the time.
On the other hand, while Vietnam occasionally revolted against China beforehand, it was not until 939 that it finally became independent, after which both were generally much more concerned with regional affairs (Vietnam against Champa/Cambodia and China against Central Asia) than to declare war on each other. Hence why the Yuan and Qing only invaded
after most of (Eastern) Central Asia and East Asia was unified under one government, while the Ming did so after pushing out the Yuan. While Vietnam continued to remain culturally East Asian, its geopolitics were much more closely aligned to Southeast Asia, so it's natural that it generally refrained from conflicts with China for ~1000 years.
Geopolitically, China before the 13th century or so was much more analogous to Europe. You rarely see countries like Spain or France at war with Hungary or Poland (wars within China), let alone England or Portugal with Russia or Turkey (China and its neighbors). Going further back, conflicts with Rome and Persia were more severe than with Germania.
Maybe they remember it more becasue they are so rare? I mean, if your continent has a war every few years then it's hard to remember them all or even separate them after a while.
Not really. Of the three "wars" fought between Korea and Japan, only the Imjin War tends to be notable because the entire peninsula was devastated, while 1-2 million (out of around 7-8 million) died and tens of thousands were forcibly carried away to Japan as hostages. In comparison, the Sui-Tang's seven collective campaigns into Goguryeo are arguably studied more within Korea because of their widespread devastation and the resulting social changes, despite the fact that it was merely one "conflict" out of many.
Strangely, however, the Mongol invasions are less discussed (in Korea), despite the fact that they were arguably the most devastating (population dropped from 8-10 million to 4-5 million), but this is probably because the level of devastation was such that countless records were destroyed, making it difficult to assess the level of chaos in the aftermath.
On the other hand, Vietnam has framed its wars against China in the context of retaining independence after almost a thousand years of continuous Chinese domination, while China takes a different approach by highlighting the various atrocities that Japan had committed, most from 1937-45, including human experimentation and comfort women.