Expect smiles and perhaps even a hug or two when President Trump finally meets Kim Jong Un in Singapore on Tuesday (9 p.m. U.S. Eastern time on Monday). Just don’t expect those protecting their respective leaders to share the happy emotions.

Because for the U.S. Secret Service and the North Korean Main Office of Adjutants and the Supreme Guard Command, this meeting brings a whole lot of stress, doubts, and possibilities for disaster.

For a start, the Secret Service must consider whether the North Koreans have concocted some harebrained scheme to assassinate Trump. While such a plot is highly unlikely, Kim's unpredictability means that the Secret Service cannot rule it out entirely. The Secret Service must also assess whether a psychopath infiltrator in Kim's ranks intends to assassinate Trump. Or whether a member of Kim's security detail is working for coup plotters in Pyongyang and has been ordered to assassinate the two leaders. Or whether any of Kim's bodyguards are Thomas Becket-style aspirants to assassination.

[Also read: Kim Jong Un brings portable toilet to Singapore so sewer divers can't examine his waste]

Still, the Secret Service's greatest fear will be a breakdown in communications or a misunderstanding with the North Koreans. Protocols governing interactions between the security details of two world leaders are always complicated, even in the U.K., and if broken, can spark problems.

Here are a few examples.

During February's Winter Olympic games, Vice President Mike Pence's Secret Service detail scuffled with South Korean officials who were harassing U.S. journalists. In 2004, then-President George W. Bush's detail fought with Chilean security officers who tried to prevent the detail from escorting Bush inside a summit. The Secret Service has also faced sustained difficulty with their unprofessional Turkish counterparts.

To avoid these situations in Singapore, the Secret Service will have worked closely with the State Department's Diplomatic Security Service, which has experience protecting North Korean security officials visiting the United Nations in New York City. The Secret Service will also likely rely on the Singapore's excellent domestic security service to act as a mediator and security guarantor for both sides.

And at Secret Service headquarters in Washington, the hope will be that all these concerns turn out to be for nothing. That outcome is very possible. After all, while the KGB and FBI grappled aggressively with each other throughout the Cold War, the KGB's 9th directorate (protective security) and the Secret Service maintained a highly constructive working relationship. Each side respected the other's professionalism and knew that cooperation was the best means of ensuring mutual safety. Their nations might have been enemies, but at the level of protective security, the KGB and Secret Service were de facto allies.

Let's hope the same is true of the North Koreans and the Secret Service.