In the letter, obtained by CNN and first reported on by Foreign Policy
, Elizabeth Shackelford wrote on November 7 that if Tillerson is unable to show the leadership required to lead the agency and reverse an exodus of diplomats like her, "I would humbly request that you follow me out the door."
Shackelford, who served in South Sudan, Kenya and Poland, wrote that under Tillerson, the State Department is being "diminished" and its influence around the world undercut as the administration increasingly relies on the military at a time of perilous foreign policy challenges.
"We have ceded to the Pentagon our authority to drive US foreign policy," Shackelford wrote, "at the behest of the White House, but to our detriment as a nation."
President Donald Trump has made military generals his closest advisers, including Defense Secretary James Mattis, National Security Adviser H.R. McMaster and Chief of Staff John Kelly, while publicly deriding Tillerson's diplomatic efforts.
Shackelford, who was most recently the Nairobi-based political officer for the US mission to Somalia, would have had a front-row seat to observe the ways that US policy in Africa is increasingly dominated by the military.
The Pentagon is spending more on the continent and putting more boots on the ground, particularly in Somalia
, where the administration hasn't replaced the ambassador who resigned in September. And while military leaders are given the freedom to meet with Somali political leaders at the presidential palace, diplomats say their requests to do the same thing are rejected on security grounds.
The military's ascendancy in US foreign policy making will only accelerate, Shackelford wrote, under Tillerson's "redesign" of the department, which he is set to unveil more details about in an address to State Department staff on Tuesday.
Tillerson announced on his first day on the job that he'd be restructuring the oldest US government agency, later revealing that the administration aimed to pare it down by around 30%.
He has come under withering bipartisan criticism
for the redesign, with senators expressing "deep reservations" about his staffing decisions and warning that the "high-level decapitation of leadership" at the State Department has "put our country in danger."
Meanwhile, consistent reports of tension
between the top US diplomat and Trump have led to persistent rumors that the White House will soon oust the former Exxon Mobil CEO.
"Despite the stinging disrespect this Administration has shown our profession," the State Department's diplomats "continue the struggle to keep our foreign policy on the positive trajectory necessary to avert global disaster in increasingly dangerous times," wrote Shackelford, an award-winning foreign service officer.
"With each passing day, however, this task grows more futile, driving the Department's experienced and talented staff away in ever greater numbers," she wrote.
"In these challenging global times, we should be seeking to conduct more diplomacy with more resources, not less," Shackelford wrote. She pointed to the absence of leadership at the top levels of the Department, saying it had been "gutted, leaving few empowered to make hard foreign policy decisions."
Shackelford pointed in particular to what she called the administration's and Tillerson's seeming indifference to human rights, highlighted in a meeting with State Department employees in May.
"President Trump's dismissive attitude toward human rights was no surprise following his campaign," Shackelford wrote, "but your May 3 remarks
to Department staff shocked many as you called into question the utility of advancing human rights
when it proves inconvenient."