Well over a year into U.S. President Donald Trump’s tenure, the State Department is in disarray. Former Secretary of State Rex Tillerson embarked on a zealous drive to reform the department as the White House proposed steep budget cuts. But what he called a much-needed push to trim and streamline a vast bureaucracy, many critics saw as a hollowing out of the U.S. diplomatic corps. Dozens of key positions, including 38 ambassadorships, remain unfilled — leaving the delicate art of diplomacy in too few hands with too many world crises at the boiling point.

With so many empty posts, the State Department is relying on lower-level officials to pick up the slack, even in embassies of strategic importance. The State Department claims it has a cadre of talented career diplomats filling the gaps in interim roles. But the stand-ins lack the clout of formal ambassadors, who are presidentially nominated and Senate-confirmed.

Most ambassadors may not be household names, but they are crucial to diplomacy, serving as the go-betweens for foreign governments and the U.S. government. Typically, about a third are political appointees. The rest are career diplomats. Their impact varies: Some come and go without making a splash. A few have caused headaches for Washington with culturally insensitive gaffes and missteps. Others are diplomatic rock stars, boosting U.S. influence and prestige abroad, and serving as consiglieres to the president. Foreign leaders take notice when the top U.S. post in their countries sits empty for too long. Here are some of the most conspicuous absences.

Mexico
An immigration crackdown, threats to withdraw from the North American Free Trade Agreement, and, of course, that wall: Perhaps no foreign country has borne the brunt of Trumpism more than Mexico. On March 1, the U.S. ambassador, Roberta Jacobson, tendered her resignation. When she formally leaves the post in May, she will join an exodus of capable career diplomats. Her resignation letter contained few details, but given the sorry state of U.S.-Mexico relations under Trump, it’s not hard to intuit her motivations.

European Union
The United States’ largest trade partner, the EU represents more than 500 million citizens. Thanks to political fissures (Brexit), growing populism, and Trump’s threat to withdraw from the Iran nuclear deal, tensions with Washington are growing. In March, the Wall Street Journal reported the White House was considering tapping Trump donor and hotel magnate Gordon Sondland to be ambassador to the EU, but it has yet to formally announce the nomination.

Turkey
Though Trump appears to admire Turkish President Recep Tayyip Erdogan’s lock on power, U.S.-Turkey relations have taken a nosedive. Erdogan has alienated Western allies 
with his autocratic power grab and crackdown on civil society. In January, Ankara and Washington teetered on the precipice of a military showdown through their proxies in Syria, where the United States backs the Kurdish forces Turkey sees as its mortal enemies.

South Korea
Seoul is squeezed in a nuclear standoff between Pyongyang and Washington. The 2018 Winter Olympics in South Korea helped dissipate tensions. On March 8, Trump agreed to an unprecedented meeting with North Korean leader Kim Jong Un. But negotiations haven’t started yet, and tensions could still flare up. Seoul isn’t just any old ally: The United States is treaty-obligated to defend South Korea and keeps some 30,000 troops in the country as a bulwark against North Korea.

Saudi Arabia
Riyadh, one of Washington’s top Middle East allies, may be basking in the glow of attention from Trump and his son-in-law-turned-senior advisor Jared Kushner. But the Persian Gulf state has run afoul of many other U.S. officials and experts for its disruptive diplomatic spat with neighboring Qatar and its bungled war in Yemen, which has left more than 10,000 people dead.

South Africa
South Africa is one of Africa’s most powerful economies and diplomatic players. Washington often drives its larger sub-Saharan Africa policy through Pretoria. The ouster of scandal-ridden President Jacob Zuma in February has plunged the country into uncharted political waters.

Sources: American Foreign Service Association, U.S. State Department

Robbie Gramer is a staff writer at Foreign Policy. This article originally appeared in the April 2018 issue of  Foreign Policy magazine.