What is the reasoning for choosing Singapore to host the North Korea–United States summit?
There are many other neutral places as far as I can tell, e.g. Japan.
So why Singapore, of all places?
The BBC ran an article outlining why Singapore was chosen. The reasons are:
Here are some more points, albeit rather "informal", from CNN
Location, location, location: It's just close enough for Kim to fly relatively easily from Pyongyang.
Ties to the US: Singapore is also one of Washington's closest Asian security and trading partners.
The embassies: The US and North Korea both have embassies in Singapore.
Trading partners: Both countries trade Singapore. But North Korean trade is currently suspended because of sanctions.
Its reputation: Singapore also has a growing reputation for being a hub of regional diplomacy.
Protesting? Think again: Singapore does not tolerate disorderly protests and rowdy press conferences.
Its neutral ground: This is key for the US.
And some excerpts from the Council on Foreign Relations:
The city-state’s diplomatic corps and security and intelligence personnel are highly respected globally and shown repeatedly that they can host a major summit without allowing any significant security or intelligence slip-ups...
Singaporean officials also have handled, many high-profile bilateral meetings, like the meeting in 2015 between Chinese President Xi Jinping and then-Taiwanese President Ma Ying-jeou. The country has an extensive array of hotels and other facilities whose staff are used to preparing for major events with tight security. The wealthy city-state also has said that it will assume some of the costs of the summit, a bonus that some other possible choices like Mongolia would not have been able to add...
The city-state also is much closer physically to North Korea than other potential sites like Switzerland or Sweden, which makes it easier for the North Koreans to travel...
Just as importantly, Singapore—like a number of countries in Southeast Asia—long has maintained ties with North Korea, as well as close links to the United States...
China and Singapore also have longstanding, if sometimes wary, relations, and Beijing probably preferred Singapore to a summit in Mongolia, Sweden, or Switzerland...
Meanwhile, although Singapore is not an official U.S. treaty ally, it is probably, at this point, the United States’s closest security partner in Southeast Asia, as well as a major trading partner. U.S. officials, throughout multiple administrations, generally have a high degree of trust in Singaporean intelligence, political leaders, and diplomats, and have worked closely with Singapore on a wide range of strategic issues. In addition, as some other commentators have noted, choosing Singapore reduces the expectations (slightly) of the summit, making it a (slightly) more low-key affair than if the two leaders had met in the DMZ or North Korea where the summit would have been even more dramatic.
And, given the nature of this meeting (and both countries' administrations), there is going to be a lot of behind the scenes reasons we may never know, so there's absolutely some subjectivity to be considered.
There's a few other points - firstly, Singapore has had a ton of experience with high profile events - ranging from IMF meetings, to the F1 rally. We've also hosted talks between the People's Republic of China, and Taiwan - two countries/states that... kind of disagree on a good many things.
Due to a quirk of history, both pre and post independance, the legal framework makes messy protests pretty much impossible, since any political gathering, or one of more than 5 people's unlawful. This makes for excellent optics. We can quietly detain protesters, anti trump or anti kim ones, and either deport or hold them temporarily. I don't think that's happened as far as I know, but it would literally take a personal appeal from one of the leaders to allow protests.
We also have an excellent, and rather understated security apparatus and the areas both leaders are in, and will be meeting on can be locked down.
While better than it used to be, the actual meeting place is basically on an island with one road, a light rail link and a cable car system, so is relatively easy to lock down in case of an emergency. At worst, there's nearby former naval facilities which the Coast guard owns. In short, while the leaders are staying in the heart of the shopping district, they can meet somewhere quiet, scenic and undistrubed.
In addition, Singapore's always been a solid but unthreatening ally to the americans, and have had solid ties with china - culturally as a state with a chinese majority, in a region that's not, and economically since they opened up.
Unlike Japan, we didn't invade any of the countries involved - and yes, there's parts of east asia that still remember WWII and its horrors, and even folk of my generation are a little sore. We also don't host any major US bases, and certainly none that the North Koreans would worry about.
In a sense, we're friends with both, but a junior partner in many ways. We're mostly harmless.
There's also what's in it for us - cause chances are we volunteered for it
Pre UN sanctions and currently, we've been happy to do business with the north koreans - if the UN lifts sanctions, Singapore would happily trade with them, and have a first mover advantage (and maybe benefit more than the 20 million we're footing for this). The North knows this, and outside China and the South, we're folks they want to be friends with.
It's also about legacy. Our current prime minister is planning on succession, and hasn't been the looming presence his father was. By facilitating something that's been in the works a while and acting as a go-between, it might cement PM Lee's legacy on the world stage.
An additional auxiliary point (the main ones are covered in the previous answers), Singapore is willing to pay for it (About $20 million) in costs. This is important as it doesn't make it seem like either North Korea or the US are paying for the other party to be there, which would be embarrassing if no agreement turned up. Also North Korea would likely have diminished ability to pay for such expenses compared to the US.
Here are some good reasons as stated in National Public Radio :
It's a neutral player in the region and both the U.S. and North Korea have embassies there. Singapore's diplomatic relations with North Korea stretch back four decades. American diplomats also work closely with Singapore on many strategic issues, and it has been a longtime U.S. trading partner and ally.
Singapore has also played host to many other bilateral meetings, and is increasingly becoming a hub for regional diplomacy in Asia. In 2015 the presidents of China and Taiwan met face-to-face for the first time in Singapore, and earlier this month, it hosted U.S. Defense Secretary James Mattis and his counterparts from the Indo-Pacific region for the Shangri-La Dialogue, an annual gathering of security officials and academics.
Tightly controlled Singapore is attractive because it is safe and unlikely to be marred by protests or security threats, as it tightly controls public demonstrations, and its security and intelligence apparatus is considered one of the best in the world. It is often ranked as one of the safest places in Asia.
Though small, Singapore is also wealthy, and able to shoulder the cost of the historic summit, which would appeal to cash-strapped North Korea.
Also location is a big factor, mainly for the North Korean leader.
EDIT: TLDR : Singapore has good trading and political relations with both countries, it has been holding other bilateral meetings recently, one of safest places in Asia, locational advantages, and is taking care of the cost of summit.