Career diplomat Matthew Bryza's departure from Baku comes one year after U.S. President Barack Obama bypassed lawmakers and temporarily installed him in the post in a 'recess appointment.'
The White House's 2010 nomination of Bryza was opposed by Armenian-American lobbying groups and several senators, who accused Bryza of having a pro-Azerbaijani bias and questionable ties to Azerbaijani officials.
Obama's move overruled an attempt by two senators, Barbara Boxer (Democrat-California) and Robert Menendez (Democrat-New Jersey), to block Bryza's confirmation.
But the appointment was only valid for the current session of Congress, which ends next week. To stay in Baku, Bryza needed the confirmation of the Senate, which went into recess before Christmas.
Bryza has more than 25 years of experience as a U.S. diplomat and was one of the most visible U.S. officials in the Caucasus region during George W. Bush's administration, serving as deputy assistant secretary of state for European and Eurasian affairs.
He is a former U.S. co-chair of the Minsk Group, which seeks to broker a settlement to the dispute between Armenia and Azerbaijan over Nagorno-Karabakh -- the region that broke away from Azerbaijani rule after a war in the 1990s and is populated mainly by ethnic Armenians.
To mark the end of his appointment, Bryza met with Azerbaijani President Ilham Aliyev on December 27 and on December 28 held a press conference to look back on his brief tenure.
He said that under his leadership, the United States "has done good work in establishing strategic partnership and friendship between the U.S. and Azerbaijan," and "deepened cooperation" on Afghanistan.
"Over one-third of all of the non-lethal equipment -- fuel, clothing, and food -- that is used by our soldiers in Afghanistan travels right through Baku, meaning on the ground, through the port of Baku, in addition to what flies over. So Azerbaijan has emerged as a crucially important transportation route for supporting operations in Afghanistan, and that route is now becoming a very important commercial asset for Azerbaijan," he said.
Bryza said that Baku and Washington have "energized an enormous range of cooperative programs" between the two countries' militaries, including improving security in the Caspian Sea and development of Azerbaijan's coastguard.
He also said Azerbaijan had witnessed an "incredibly successful year" in its energy sector, highlighted by the signing in late October of an agreement between Baku and Ankara to open the so-called Southern Corridor for natural gas deliveries to Europe.
Bryza said he had also pressed the Aliyev government on its troubling human rights record.
"We're all familiar with the criticism by myself, my government, [and] the West about shortcomings on democratic reform and human rights [in Azerbaijan] and the areas where a lot more should be done and I hope will be done," he said. "We repeatedly have expressed our views in public, and worked hard in private, to create the opportunity for people to express their views freely."
In that spirit, he took one last opportunity at his press conference as the country's top U.S. official to call on Baku to ensure that journalists and protesters currently in jail are given due process and their legal rights.
The Aliyev government has cracked down on activists and opposition leaders in the wake of mass demonstrations last spring that organizers say were inspired by recent popular uprisings in the Arab world.
But Bryza said today that he doesn't think that an "Azerbaijani Awakening" will take place, in part because better economic conditions exist in the country than in most Arab countries. He also said "very enlightened officials" in the government understand the need to liberalize.
News of Bryza's departure has been welcomed by members of the Armenian-American community. Aram Hamparian, who leads the influential lobbying group Armenian National Committee Of America (ANCA), is among them.
"U.S. diplomacy in this part of the world needs a fresh start. We need a new ambassador who can show up without any bias, without any baggage, and start fresh. We would put a very high focus on challenging very assertively the Aliyev government," he said.
But Bryza has powerful supporters, as well. Fred Hiatt, the editorial page editor of the influential "Washington Post" newspaper, wrote recently that Bryza's departure is a "vivid example of how the larger U.S. national interest can fall victim to special-interest jockeying and political accommodation."
More than 30 policy experts and former U.S. government officials sent a letter to Congress earlier this month urging members to keep Bryza, whom they called "an exemplary ambassador," in Baku.
Until a new U.S. envoy to Azerbaijan is nominated and confirmed, the post will likely be filled by the embassy's current deputy chief of mission, Adam Sterling.
with additional reporting from RFE/RL's Azerbaijani service