Galina Timchenko, publisher and chief executive of the investigative newsroom Meduza, thought she was ready for anything. The site, based in Latvia and known for its fearless reporting on Vladimir V. Putin’s regime, had prepared for cyberattacks, legal threats and even poisonings of its reporters.

One thing she hadn’t anticipated: defunding by the U.S. government.

Meduza, which had received roughly 15 percent of its annual budget from programs funded by the U.S. government, has been thrust into a financial crisis after the Trump administration abruptly stopped all foreign assistance from the United States Agency for International Development and other federal agencies this month.

“U.S.A.I.D. or the State Department, usually they fulfill their obligations. They follow their rules,” Ms. Timchenko said. “Now, it’s some kind of a broken world.”

Meduza is one of hundreds of newsrooms in dozens of countries that until now benefited from at least $180 million in annual funding to support journalism and media development from U.S.A.I.D., the State Department and the National Endowment for Democracy, a government-funded nonprofit. The decision has already forced cutbacks, layoffs and long-term uncertainty for many independent newsrooms.

“It’s really a blood bath,” said Anya Schiffrin, a senior lecturer at Columbia University specializing in international nonprofit media and investigative reporting. “These are the only journalists who are holding governments to account in many parts of the world, and without U.S. support there’s just not a lot of other money available.”

The U.S. government has been the world’s largest supporter of independent foreign media, principally through U.S.A.I.D., since the early 1980s. The funding is meant to foster democracy through transparency, as part of the country’s larger portfolio of soft power efforts. It has helped finance some of the most consequential investigative journalism of the past decade, including the Panama Papers, which won a Pulitzer Prize for uncovering international money laundering, and the FinCEN Files, which showed how banks facilitated corruption around the world.