Haiti, a nation rocked by gang cruelty and plagued with political infighting, has — so far this year — had three prime ministers, seen at least 4,000 people killed and experienced brutality from armed groups so intense that it forced an extended closure of its international airport, twice.

But despite $600 million spent by Washington on an international police force to restore order, an explosion of violence last week underscored the enormity of a crisis so severe that the Federal Aviation Administration has barred U.S. aircraft from flying under 10,000 feet in Haitian airspace to avoid being shot at by gangs.

With another interim prime minister in place, but gangs gaining territory every day, Haitians are desperate for relief. Efforts to stabilize Haiti are floundering, and the country presents a dangerous and disastrous challenge as President-elect Donald J. Trump prepares to take office.

Few people seem to have answers.

“I am a complete loss,” said Susan D. Page, a University of Michigan Law professor and former United Nations official in Haiti. “Everyone is just kind of astounded.”

What’s going on?

Haiti has experienced a long-simmering crisis for about 15 years, a period marked by a devastating earthquake, squandered aid dollars, tarnished international interventions and flawed presidential elections.