After less than two years on the job, Nikki Haley is planning to step down as US ambassador to the United Nations. Her relatively short tenure isn’t unusual for the position — she has lasted longer than half of her ten predecessors — but her decision still caught Washington by surprise.

There was immediate speculation as to why Haley is stepping down, most of it focusing on a potential run for office.

But Haley told reporters at the White House, where she announced her resignation next to Donald Trump, that she would not be running for president in 2020 and would support Trump. (She did not mention recent allegations of financial impropriety that were made public just hours before by an ethics watchdog.)

Whatever made Haley decide to “take a little time off”, the consequences of that decision will be watched closely around the world.

But when it comes to actual changes in US foreign policy, the difference may be more in the style than the substance. That doesn’t mean that Haley wasn’t an important figure. Despite Trump’s frequent criticism of the United Nations, he followed the policy of the Clinton and Obama administrations and made the US ambassador to the United Nations a full Cabinet-level position.

When Trump’s first secretary of state, Rex Tillerson, proved media-shy, Haley became the de facto voice of the administration’s foreign policy.

To many establishment Republicans, Haley was one of only a handful of “adults in the room” when it came to foreign policy, promoting the idea that “America First” didn’t have to mean “America alone”.

She privately opposed some administration policies, such as a recent reduction in the number of refugees resettled in the United States, and took a harder public stance on Russia than her boss.

Some people even posited that Haley may have been the infamous “senior official” in the Trump Administration who penned a New York Times op-ed saying that many people within the executive branch were quietly trying to contain Trump’s worst impulses.

But if Haley was that unnamed official, there’s little evidence that Trump himself suspected it.

At the White House on Tuesday, he lauded her accomplishments and said she had made the UN ambassador position “glamorous”. Indeed, by most accounts, Haley had a warm relationship with the President. Trump had good reason to value Haley.

She personally supported many of his more controversial foreign policy moves, including recognising Jerusalem as the capital of Israel and moving the US Embassy there from Tel Aviv and withdrawing the United States from the UN Human Rights Council.

She also supported Trump’s push to cut US funding to countries unwilling to support American foreign policy and took a hard line on foes such as Iran and Venezuela.

Haley replied to Trump’s kind words about her in similar vein, praising the President and his family with Trumpian flair: Presidential son-in-law Jared Kushner is “a hidden genius that no one understands”, she said, and the United States is now “respected” thanks to Trump.

Still, as The Washington Post’s Anne Gearan and John Hudson reported last month, Haley had taken on a less important role in recent months.

Tillerson’s replacement as secretary of state, Mike Pompeo, promised to reinvigorate the State Department with “swagger” and took on much of the diplomatic grunt work when it came to North Korea and Iran, arguably Trump’s two most important foreign policy problems. Trump also hired a new national security adviser, John Bolton, who is both a former US ambassador to the United Nations and a famous critic of the institution.

Bolton possesses a detailed understanding of the way the institution works — and has his own agenda for how to undermine it.

Whatever else may have prompted Haley’s decision to step down, it seems likely that one factor was that she was no longer so much needed as the Trump Administration shuffled its staff.

Haley’s next step isn’t clear. “She’s running for something — we just don’t know what,” is how The New Yorker’s Susan Glasser put it.

While a 2020 run for president is apparently not in the cards, she probably is considering something further down the line.

Her short stint at the United Nations has given her crucial foreign policy experience that she had not gained in her previous careers in business and politics.

It’s also not clear who will take Haley’s place, though Trump says he already has a few names in mind and will announce the new ambassador in a few weeks.

Among the names floating about: Dina Powell, Marco Rubio, Richard Grenell, Tom Cotton or even the President’s daughter, Ivanka. Another popular theory is that Haley would take over the seat of Lindsay Graham, the senator from South Carolina, while he took hers at the United Nations — or perhaps that of embattled Attorney-General Jeff Sessions.

In this administration, anything seems possible. But it’s hard to imagine any of these successors changing the course of US foreign policy on major issues.

Instead, it seems that the next UN ambassador will have even less political independence or media visibility than Haley enjoyed.

But did that independence ever really exist at all? Haley is a confident, intelligent and politically ambitious person who had sparred with Trump before he was elected.

Yet her time at the United Nations still mostly consisted of pushing through Trump policies that alienated the United States from the rest of the world.

Her critics will not remember her as an “adult in the room” but as an enabler of Trump’s frequently brutal foreign goals.

• Adam Taylor writes about foreign affairs for The Washington Post. Originally from London, he studied at the University of Manchester and Columbia University