BRUSSELS — In the alliance-disrupting era of President Trump, America’s longest-standing friends are nervous about whether the United States would defend them in a conflict. They don’t know whether the White House prefers democratic leaders or autocrats. And the European Union’s ambassador to Washington can’t even count on getting a decent spot in the reception line.

The State Department’s office of protocol downgraded the European Union in its order of precedence, E.U. officials said Tuesday. That relates to where diplomats are seated at dinners, when they are invited to receptions and in what order they are called to pay respects at state funerals, such as that of former president George H.W. Bush last month. No longer will the European Union be as exalted as equivalent to a country. Instead it will be back alongside the African Union, after the rest of the national ambassadors.

The change comes as relations are strained between Washington and Brussels. Trump has hammered the European Union for what he says are unfair trade practices. He congratulated Britain for voting to quit the bloc and criticized British leaders when they seemed too conciliatory toward Brussels. And he pulled out of the Paris climate agreement and Iran nuclear deal, two accords prized by the Europeans.

Last month, Secretary of State Mike Pompeo blasted international organizations in a wide-ranging foreign policy speech in Brussels that many in his audience took as a declaration of war.

The next day, E.U. Ambassador David O’Sullivan wasn’t called in the ordinary order at Bush’s funeral and suspected something was amiss, an E.U. official said.

Since President Barack Obama upgraded the European Union’s status in 2016, O’Sullivan had been ranked among national ambassadors based on length of tenure in Washington. O’Sullivan is relatively senior, having been posted in 2014. (The longest-posted ambassador in Washington? Hersey Kyota, of the Pacific island nation of Palau, who presented his credentials in 1997.)

The European Union was never formally notified of any downgrade and inquired about it after the funeral. O’Sullivan attended at least one subsequent event where again he seemed to be treated as a national ambassador, leading to some confusion, an E.U. official said, speaking on the condition of anonymity to candidly discuss cocktail reception sensitivities. But when E.U. officials searched for the protocol list on Google — often the best way to obtain information in the Trump era — there it was: the European Union, all the way at the bottom, as of Oct. 30.

“We are currently discussing with the relevant services in the administration possible implications for the E.U. delegation in Washington,” Maja Kocijancic, a spokeswoman for the E.U.’s diplomatic service, said Tuesday.

A spokesman for the U.S. Mission to the European Union declined to comment, saying that the U.S. government shutdown was affecting the State Department’s ability to respond publicly.

The change was first reported by German broadcaster Deutsche Welle.

Some European diplomats said they didn’t rank it as particularly important, especially given the long list of substantive disagreements between the two sides. An E.U. official said they would try to move onward without further fuss.

But some analysts were quick to add it to the list of slights from the Trump administration. Even if the protocol change was purely symbolic, they said it showed that there are few arenas of relations left untouched by Trump’s disdain for the European Union.

“Something like this, even if it was a protocol snafu, runs the risk of being seen as a more deliberate offense or a sign of disrespect for the importance of the political role by the E.U.,” said Amanda Sloat, a former Obama administration official and a Europe expert at the Brookings Institution. “Anything being done by the administration that seems to downgrade or denigrate in any way the European Union is cause for concern and interpreted negatively.”

The downgrade has little direct effect on the European Union’s ability to operate in Washington, although it might indirectly curtail its influence by slimming it from invitations to certain events and giving O’Sullivan a less prominent place at State Department-organized dinner tables.

“The administration doesn’t want to actively support the E.U., and so they are probably looking for ways to adopt what they would consider a more neutral stance. This may very well have struck someone as a way to move in that direction,” said Thomas Wright, a senior fellow at Brookings. “In Europe, this will be perceived as yet another step to actively undermine the E.U.”

Hudson reported from Washington.