The unannounced move by the US State Department, which has not previously been reported, downgraded the EU delegation's diplomatic status in Washington from member state to international organization.
"We don't exactly know when they did it, because they conveniently forgot to notify us," an EU official who is familiar with the matter told DW in an interview.
"I can confirm that this has not been well received in Brussels," the person said, adding that the issue and an official EU response was still being discussed.
After the delegation noticed that the EU's Washington ambassador had not been invited to certain events late last year, officials organizing the state funeral for President George H.W. Bush provided final confirmation to EU diplomats that the status of the representation had in fact been downgraded. Diplomats believe the downgrade must have been implemented in late October or early November.
At the high-profile event on December 5, as diplomats gathered in Washington to pay their respects, O'Sullivan was not called up in the usual chronological order from the longest-serving to the newest ambassador, said the EU official. "But he was called up as the last person."
Prior to the demotion, O'Sullivan — who has served as the EU's ambassador to Washington since 2014 — would have been ranked among the first 20 or 30 ambassadors of the more than 150 foreign representatives dispatched to the US capital.
'Forgot to notify us'
A Washington-based diplomat of an EU member state also confirmed the downgrade and denounced the move.
"This is clearly not simply a protocol issue, but this is something that has a very obvious political motive," the person said. The diplomat added that the negative view of the EU mission downgrade was shared by the majority of member states.
After discovering the downgrade, EU diplomats in Washington reached out to the State Department, which is responsible for diplomatic affairs, for clarification. "They have told us that they forgot to notify us and that this is a decision they have taken because that is apparently what the chief of protocol thinks is the proper thing to do," the person said.
The EU official said the bloc ambassador's status in Washington was upgraded to the level of a nation-state ambassador in September 2016, after a lengthy and intensive process by the State Department under former President Barack Obama.
"For us, they have made this decision once and it was a well-thought-through decision — and now they sort of put it down again with a pen stroke," the EU official said.
The person added that while it's not uncommon for a new administration to rethink protocol classifications, it's unusual that such a downgrade happens after a new administration has been in office for nearly two years. It's also rare that a diplomatic mission is not informed by the host country about such a decision ahead of time, and in writing.
"I have big difficulties to see that this has anything to do with a new administration or a new protocol, because time-wise it's really very far removed," the official said.
The diplomatic downgrade of the EU's mission in Washington appears to be in line with what is widely being perceived as an anti-EU stance by the Trump administration.
Trump was an avid supporter of Britain's exit from the European Union during his presidential campaign, and as president has repeatedly lashed out at the European Union on issues such as trade and defense.
State Department stands by its decision
In a major foreign policy speech in Brussels in early December, Secretary of State Mike Pompeo praised Trump, a self-declared nationalist, and urged European countries to reassert their national sovereignty vis-a-vis the EU.
Meanwhile, back in Washington, in an effort to try to reverse the protocol downgrade, EU diplomats contacted the State Department but were ultimately told just before the Christmas holiday that the department stood by its decision, the EU official said. "But we have not received anything written."
Asked for comment, the State Department did not respond on the issue, citing limited operations due to the government shutdown. "Due to the lapse in appropriations, the Press Office will be operating on a reduced status. Communications with the media will be limited to events and issues involving the safety of human life or the protection of property, or those determined to be essential to national security," the State Department said in an email.