WASHINGTON — When Kim Jong-un, the North Korean leader, met with top Chinese and Russian officials before a planned summit meeting with President Trump, the American leader raised a red flag. But North Korea’s announcement that Mr. Kim is also planning to meet President Bashar al-Assad of Syria has brought no similar protest from Mr. Trump — even though Syria and North Korea have a longstanding shared history of nuclear proliferation.

The last time Mr. Assad did business with the Kim family, the result was one of the most brazen cases of proliferation in history: North Korean engineers built a replica of their main nuclear reactor in the Syrian desert. It was the beginnings of a nuclear program that ended in fiery ruins in September 2007, when the building was destroyed in a secret Israeli bombing run.

At the time, Vice President Dick Cheney and others in the George W. Bush administration argued that the United States should have bombed the reactor itself, to make a point about its seriousness in stopping the export of bomb-making technology.

Now North Korea says it is resuming the relationship, and is expecting a visit to Pyongyang by Mr. Assad — who has rarely left Damascus since the uprising against his government began seven years ago.