President Donald Trump has threatened to pull tens of thousands of US troops out of Italy, Spain and Germany amid disagreements over the war in Iran.

Trump has previously criticized NATO allies for not sending navy ships to help open the Strait of Hormuz, a thin waterway through which 20 percent of the world's oil supply passes.

He has also threatened to withdraw the United States from the alliance. 

But his rhetoric ramped up on Wednesday, after German Chancellor Friedrich Merz said the United States was being 'humiliated' by Iran and criticized Trump for not having an effective strategy to end the war.

'The United States is reviewing the possible reduction of Troops in Germany, with a determination to be made over the next short period of time,' the commander-in-chief posted to his Truth Social platform.

He has since expanded the threat to include troops based in Italy and Spain. 

'I mean, they haven't exactly been on board,' Trump said when asked about the possibility of cutting troop levels in the two countries.

'Yeah, [I] probably will,' he continued. 'Why shouldn't I? Italy has not been of any help. Spain has been horrible. Absolutely.' 

The United States has more than 36,400 active-duty US military personnel permanently stationed in Germany, according to data from the US Defense Manpower Data Center released in December.

There are also more than 12,600 active duty US military personnel in Italy and more than 3,800 in Spain. 

Removing those troops will now be difficult for the Department of Defense to execute amid the war in Iran, Politico reports.

It would also be expensive for the US government to move troops, their families and equipment back to the US, given there likely isn't available housing for them. 

At the same time, taking out American forces could remove a major military deterrent against rearming Russia, which European officials believe is preparing to attack NATO in the coming years.

But Pentagon spokesman Sean Parnell told Politico the department 'plans for every scenario and we are fully prepared to execute the orders of the commander-in-chief at the time and place of his choosing.' 

Yet officials at the Department of Defense were also reportedly left stunned by Trump's initial threat on Wednesday to pull troops out of Germany.

The threat came as Germany's Chief of Defense, General Carsten Breuer wrapped up a series of meetings with US officials in Washington to discuss Berlin's new defense strategy - which calls for the country to increase defense spending to 3.7 percent of its GDP by 2030.

It also came as US Army Secretary Dan Driscoll wrapped up a two-day trip to German training ranges to underscore US presence in the country.

Italy's Defense Minister Guido Crosetto told Italian news agency ANSA on Thursday he didn't 'understand the reasons' for Trump's threat to remove troops.

'As it's clear to anyone, we haven't used the Strait of Hormuz. And we've even offered to carry out a mission to protect shipping - a gesture that was actually greatly appreciated by the US military,' he said, noting Italy is also providing assistance to Ukraine amid its war with Russia. 

'The incredible thing is that they've used the Strait of Hormuz, while we don't.'  

It now seems that Trump made the threat to remove forces from Germany in response to Chancellor Merz's remarks on the war in Iran.

'The whole affair is ill-considered to say the least,' Merz said, emphasizing Berlin’s desire to see the war end as soon as possible. 

'At the moment, I cannot see what strategic exit the Americans are now opting for,' he continued, comparing  the situation to past issues in Afghanistan and Iraq.

The US president quickly lashed out at Merz on his Truth Social page on Tuesday, writing: 'The Chancellor of Germany, Friedrich Merz, thinks it’s OK for Iran to have a Nuclear Weapon.

'He doesn’t know what he’s talking about!'

Despite the rhetoric, Germany has offered limited military support to the US in the war in Iran, most notably by allowing American troops to use its airbases for operations related to the war in Iran - though it does not allow for the bases to be used as staging grounds for direct strikes, CNN reports.

German Chancellor Merz has also pledged to provide further assistance if the conflict moves into a post-war phase, including a possible international stabilization mission.

Berlin even recently announced that naval minesweepers will be deployed to help secure the Strait of Hormuz, and Germany is also part of a coalition led by the UK and France to ensure vessels cross the strait safely.

Merz sought to cool tensions with Trump on Wednesday, insisting their relationship remains strong despite the heated public argument.

'The personal relationship between the American president and me remains, in my view, as good as ever,' the German Chancellor said at a press conference. 'We are still, to put it in American English, on good speaking terms.'

He framed his recent criticism of the Iran war as a response to the conflict's economic fallout, rather than Trump’s leadership. 

'We are suffering considerably in Germany and in Europe from the consequences of … the closure of the Strait of Hormuz,' he said, citing soaring energy costs and economic strain.

Efforts to peacefully resolve the conflict in Iran have hit an impasse, as Iranian forces continue blocking the strait in response to a US naval blockade of Tehran's oil exports, the country's economic lifeline. 

Trump was due to receive a briefing ​on Thursday about plans for a series of fresh military strikes on Iran to compel it to negotiate an end to the conflict.

But a senior member of Iran's Revolutionary Guards warned that any US attack on Iran, even if limited, will usher in 'long and painful strikes' on US regional positions.

'We've seen what happened to your regional bases, we will see the same thing happen to your warships,' Aerospace Force Commander Majid Mousavi was quoted by Iranian media as saying. 

The Daily Mail has reached out to the White House and the Department of Defense for comment.