H. Beam Piper was an American SF writer, best known for the series Paratime (which is about exploiting The Multiverse for fun and profit); the Terrohuman Future History, or TFH, which is about the human race spreading throughout the stars, and the cultural rising and falling that happens as a result; and Little Fuzzy, an arc within the TFH series about the discovery of intelligent life on a settled planet. He committed suicide in 1964 because of financial problems. A check was literally in the mail.
His first published story, "Time and Time Again" (1947), launched an alternate universe when the dying main character's consciousness was flung thirty years back in time to his then-thirteen-year-old body — and decided to change history to prevent the World War Three in which he'd been killed. His plans involved having his father, Blake Hartley, become President in 1960; two later stories, set in '65 and '68, mention President Hartley, so the plan was successful to that extent at least....
Although the one about the diplomat isn't explicitly labelled a Paratime story, the first specifically Paratime story makes an apparent reference to the incident as having been accidentally caused by a Paratime policeman. At least the dates and a one-sentence description of the events match up.
Brown Note: The aliens in "Naudsonce" experience sound as physical sensations. The Terran expedition finds it necessary to bury their water pump because it gets the aliens blissed out to the point of neglecting their farms; their linguist is handicapped because her voice causes the aliens extreme discomfort.
Continuity Nod (Carlos von Schlichten and Paula Quinton have a romance in Uller Uprising; the short story "Oomphel in the Sky" has a reference to a Paula von Schlichten Fellowship, which is in sociography, Paula Quinton's field.)
The short story "Naudsonce" named a exploratory starship Hubert Penrose after an important character in "Omnilingual."
Crusading Widower: Lucas Trask in Space Viking. He slides right into What the Hell, Hero? by nuking cities and looting them to pay for all that crusading. Bonus points? He delivers the "what the hell" speech to himself.
Death World (Fenris in Four-Day Planet comes very close, if not outright qualifying)
Due to the Dead (Are the Little Fuzzies intelligent? Well, they bury their dead.)
Earth That Used to Be Better (In ''Space Viking", the main character worries about his home planet's civilization declining, and a historian agrees: "That's what happened to the Terran Federation, by the way. The good men all left to colonize, and the stuffed shirts and yes-men and herd-followers and safety-firsters stayed on Terra and tried to govern the Galaxy.")
Eternal English (Averted. Most of the TFH uses a kind of linguistic potpourri that's basically every modern language run through a blender at once.)
Fantastic Slurs (Ullerans are known as "geeks". Partially through onomatopoeia from some local languages, partially because some Ulleran cultures kill small, iguana-like food animals by biting off their heads.)
The Khooghra of Yggdrasil are officially sapient, but so stupid that calling a Terran a "son of a Khooghra" once led to a shooting. The man so described knew he was being insulted.
The Federation: The Terran Federation during the early part of the TFH
Genius Bruiser (Otto Harkaman in Space Viking, who's a talented historian and as big as a house.)
Hideous Hangover Cure (Not actually hideous or a hangover cure; the "alcodote-vitimine pill" won't let you get drunk at all.)
Homage : Traveller revises the Sword Worlds of Space Viking to suit the Traveller universe.
Horse of a Different Color (Freyan oukry, which are used to make Westerns. Most people in the TFH seem to think horses are extinct; a minor news story in Four Day Planet mentions a movie shot using real horses.)
Human Aliens (the Freyans, spelled out in the novella "When In the Course..." They're human enough to interbreed with Terrans, despite the Terran doctor insisting it's impossible. Piper apparently had some explanation in mind, most likely some variant of Transplanted Humans, but it was never revealed.) The story was Retconned out of Future History, and substantially rewritten to become Lord Kalvan of Otherwhen in the Paratime milieu.
Although Paula Quinton from Uller Uprising states that she's part Freyan, so the concept wasn't completely eliminated.
Also Martians, in the TFH: they died out 50 millennia ago, but statues, paintings, and mummified corpses in "Omnilingual" are specifically stated to look fully human. Considering the short story "Genesis" and the Paratime series both claimed Earth humans are descended from Martian colonists, Piper may very well have had this origin in mind for the TFH too.
Humans Are White : Averted. While explicit physical descriptions of characters are rare, the prevalence of multiethnic names indicates that most of them are some shade of brown.
Multiethnic Name: Particularly common in Uller Uprising, where just about every major character's given name and surname are of distinctly divergent ethnic origin. Justified by mass migration from the nuked northern hemisphere to South America and Africa following the Fourth World War.
Mundane Utility (we're going mining on Flourine-Tainted Niflheim, the Planetary Hell...volcano mining with atomic warheads.)
Nuke 'Em (ship-to-ship combat in Space Viking, the climax of Uller Uprising.)
Our Souls Are Different - The Last Enemy involved a world where reincarnation was a proven scientific fact, and the resulting cultural changes. Most important: death was considered a temporary inconvenience at worst, leading to suicide parties being a socially accepted practice and frequent duels to the death taking place.
Past Life Memories: The retrieval of these by means of mediumschanneling the souls of the recently deceased, proving one theory of reincarnation true in The Last Enemy. This leads to social chaos as people sue to reclaim property they had in former lives.
Reincarnation: Focus of the plot in The Last Enemy, with an exploration of the social effects that result in its being proven to exist, along with two political factions fighting over their rival theories about how it works.
Silicon-Based Life - Life on Uller, including four-armed humanoid reptiles and creatures like hexapodal pine cones.
Space Pirates (or, more accurately, Space Vikings. They don't board and rob ships, they nuke cities from orbit and loot any cities that chose to surrender.)
Stuff Blowing Up: Ranging from exploding bullets to fusion fireball bombs that destroy everything within a thousand miles with a miniature sun.
Technology Marches On: The TFH stories include videophones, antigravity, faster-than-light travel...and... cameras that use film which must be developed before viewing and huge computers that fill whole rooms and are programmed via plugboards.
The Masquerade: One of the primary tropes of the Paratime stories: you can go visit other universes, but you're not supposed to let the locals catch on that you're from another universe... It might cut into the 'profit' end.
Trask also gets one from a comrade after gunning down a local who was grieving for a dead spouse. His response was that he was putting the man out of his misery, and included the words: "How many more happinesses do you think we've smashed here today? And we don't even have Dunnan's excuse of madness."
He also says that he wished that Dunnan had done that for him, "so that none of this would have happened." (see Crusading Widower above.)
Writer on Board: Piper believed in reincarnation and wrote a Paratime story about a world where it was proven to the hilt. Even with this, it was still a pretty good story.
Perhaps more importantly-in that story? The problem's because reincarnation is proven to the hilt-and they've started to get too good for The Masquerade's sake at retrieving memories of past lives...