European officials were quick to congratulate Donald Trump after he defeated his Democratic rival Kamala Harris and returned to the White House, even though it was clear that another economic war could be imminent.

European diplomats and their respective leaders have been preparing for Trump's possible victory for more than 12 months, with a growing focus on measures that could insulate the European economy from possible trade disputes.

Some European officials woke up to the election results on Wednesday and “didn’t want to believe them,” multiple sources told CNBC.

“I see it and I don’t want to believe it,” said an EU official who did not want to be named due to the sensitive nature of transatlantic relations. “But I’m not as shocked as I was last time.”

Many European leaders disliked Trump's confrontational leadership style during his first presidency and there were several moments of tension with the former White House chairman. As a result, many in Brussels celebrated Joe Biden's victory in 2020 and hoped for better engagement.

A second EU source, who also did not want to be named because of the sensitivity of the relationship, said: “It's not great again.”

But the source echoed the previous official's sentiments, conceding: “At least I'm not as surprised (as I was in 2016).”

European Commission President Ursula von der Leyen, French President Emmanuel Macron, Spanish Prime Minister Pedro Sanchez, Italian Prime Minister Giorgia Meloni and Hungarian Prime Minister Viktor Orban were among the first EU leaders to greet Trump on Wednesday morning brought congratulations.

Concern about Trump is not fully shared across the European continent. Hungarian Prime Minister Viktor Orban, who has expressed his admiration for Trump in the past, was previously reported to have said he would open a bottle of champagne if Trump were re-elected.

EU leaders will meet in the Hungarian capital Budapest on Thursday and Friday for a regular meeting that will give them the opportunity to discuss their future plans for transatlantic relations.

Trump has threatened to impose additional 10% tariffs on European countries while declaring that the European Union will “pay a heavy price” if it doesn't buy enough American goods.

Trade with the United States is vital to European nations. The EU and the US have the largest bilateral trade and investment relationship in the world, reaching an all-time high of 1.2 trillion euros ($1.29 trillion) in 2021, according to the European Commission, the EU's executive body.

Any additional tariffs could put further pressure on already weak economic growth across the EU.

“There will be a first discussion (on the outcome of the US elections) in Budapest,” a third anonymous EU source told CNBC on Wednesday morning about the European Political Community (EPC) meeting that will start on November 7.

“We will see what rhetoric (Trump) will have, but unless it is very different, Ukraine will be the biggest challenge,” the third diplomat said. “Our main focus is on maintaining European unity,” the same diplomat said.

In a research note To On Wednesday morning, ING analysts said that with Trump's re-election, Europe's “worst economic nightmare” had come true.

“A looming new trade war could plunge the euro zone economy from sluggish growth into a full-blown recession. The already struggling German economy, which relies heavily on trade with the US, would be hit particularly hard by tariffs on European automobiles,” he told the analyst team led by James Knightley.

“Although European politicians claim they are prepared for a second Trump presidency, it remains unclear whether Trump could actually spur deeper integration, given the domestic challenges facing many European governments. Europe will probably wait and see what measures Trump actually implements.”

At the IMF annual meeting in Washington, DC last month, German Finance Minister Christian Lindner warned that there could be retaliation if the US started a trade war with the European Union.

“We need diplomatic efforts to convince everyone who enters the White House that it is not in the best interests of the United States to have a trade conflict with the European Union,” he said, adding: “We would have to think about retaliation ponder.”