It's a valid question, but it's also overly simplistic. Asking for "one thing" that exemplifies the success of U.S. international diplomacy is sort of like asking for the one reason the U.S. lost the Vietnam War. And Clinton herself has a rebuttal; The Washington Post has a good roundup of the achievements she lists in her book.
Rule #3: She's too weak!
Liberals often deride Clinton for her hawkishness, but conservatives see her as a pushover. She can't win — push too hard and she's an unrelenting harpy; try to take a nuanced approach and she wants America to be the world's doormat.
Failed Choices first links Russia's tense relations with the U.S. to the reason for the continued massacres in Syria. "The Reset hurt America's standing in the world because it backfired so badly," the authors write. "It failed to prevent innocent Syrians from being murdered by Russian weapons. It failed to protect journalists, gays, and dissidents from being jailed in Russia."
The authors argue that Clinton "heaped praise" on Russia by telling an interviewer, "We view Russia as a great power." They gloss over the fact that she went on to compare Putin to Hitler. (Putin more recently retorted, "It's better not to argue with women.")
Rule #4: "She's too aggressive (against Israel)!"
The authors of Failed Choices admit that Clinton was a "staunchly pro-Israel senator," but they argue she "became a leading critic of Israel within the administration."
Their evidence for this loss of support for Israel? Clinton backtracked on her pledge for an "undivided Jerusalem," opposed showing credible force to invade Iran, and "only" visited Israel five times as secretary of State. The authors point out that, by comparison, Condoleezza Rice made 25 trips to Israel when she was secretary of State. What they don't point out is that Rice visited 29 fewer countries than Clinton, and brokered a deal with Israel in 2005 to withdraw from Palestinian territories.
Failed Choices accuses Clinton of opposing new Israeli settlements; Clinton, meanwhile, says she "expressed quiet reservations" about Obama's proposal to stop Israelis from building new settlements. The most petty claim in Failed Choices is that unnamed Israeli officials were "unhappy about being left out" of a conference hosted in Turkey on counterterrorism.
None of these accusations exactly make Clinton a Friend of Hamas, but in 2016, Republicans and Democrats will seek succor from pro-Israel voters — and donors. It's important to keep up appearances.
Rule #5: Clinton Inc.
Painting Bill and Hillary Clinton as a well-heeled political dynasty is nothing new. But in Failed Choices, the authors go beyond that depiction, calling Hillary Clinton a "salesperson for select U.S. business interests" by using "economic statecraft."
The authors highlight a few shady deals, implying pay-to-play. Huma Abedin, a longtime Clinton aide (and Anthony Weiner's long-suffering spouse), did not disclose that she worked for the consulting firm Teneo while she was still working for Clinton. Though no real foul play came to light, conservatives would love for Teneo to become for Clinton what Solyndra was to Barack Obama, or GreenTech Automotive was to Virginia Gov. Terry McAuliffe (a close friend of the Clintons). Still, it's good to remember that both Obama and McAuliffe won their respective elections.