In the weeks leading up to a critical annual U.S. report on human trafficking that publicly shames the world's worst offenders, human rights experts at the State Department concluded that trafficking conditions had not improved in Malaysia and Cuba. And in China, they found, things had gotten worse.

The State Department's senior political staff saw it differently — and they prevailed.

A Reuters examination, based on interviews with more than a dozen sources in Washington and foreign capitals, shows that the government office set up to independently grade global efforts to fight human trafficking was repeatedly overruled by senior American diplomats and pressured into inflating assessments of 14 strategically important countries in this year's Trafficking in Persons report.