The United States is reportedly considering a significant reduction in its military presence in Europe after its allies there refused to allow Washington to use their airbases for the war on Iran.

US media reported this week that senior Pentagon commanders are expected to submit plans to US Defense Secretary Pete Hegseth in the coming days.

The news comes amid Russia’s ongoing war on Ukraine and as European countries increasingly sound the alarm about possible sabotage operations on their territory; Germany accused Russia of a drone attack on an airport earlier this month.

Europe has, for decades, depended on its partnership with Washington via the North Atlantic Treaty Organisation (NATO) to deter a possible Russian attack. However, relations between some European capitals and Washington have deteriorated in the past year.

Here’s what we know about what options the US could be considering:

Why is the US reviewing its presence in Europe?

The Pentagon is weighing several possible options for reducing troops and military assets in Europe as part of a NATO review it announced in June, according to NBC News, which cited security sources.

Hegseth alluded to the review at a NATO meeting in Brussels in June, when he lashed out at fellow members and said new changes would be “designed to ensure that NATO is moving fast and irreversibly toward Europe leasing and stepping up to take primary responsibility for the defence of Europe”.

Hegseth said European allies’ decision not to provide US forces with access to bases in Europe to launch attacks on Iran was “shameful”.

“These allies, they put America’s sons and daughters, our sons and daughters, at risk by denying them the predictable access, basing and overflight that never should have been in question at all.”

What options is Hegseth considering?

One option could be to withdraw one-third of the estimated 68,000 US troops stationed in Europe. This option would largely affect soldiers stationed in Germany, Italy and Spain, NBC reported.

A second, more drastic scenario could see 40,000 soldiers exit, along with military aircraft, ships, and weapons.

The US may also consider moving troops away from stations in Central or Western Europe to NATO’s eastern flank, closer to Russia.

Hegseth is expected to reach a final decision by November 6, NBC reported.

Has the US already withdrawn troops from Europe?

Yes. About half of the 2,000 US soldiers stationed in Romania departed last year.

Some 5,000 US troops are in the process of withdrawing from Germany, where more than half of the European contingent is based.

The announcement of their withdrawal in May came after German Chancellor Friedrich Merz criticised the US’s war on Iran. Trump threatened to go “a lot further”, but US Secretary of State Marco Rubio reportedly blocked the White House from withdrawing as many as 25,000 soldiers at the time, according to US media.

What would a US military drawdown mean for Europe?

A mass exit of troops and assets would be significant for Europe as it would undermine the US’s nuclear deterrence posture on the continent, said Ian Lesser of the German Marshall Fund.

While Russia is not likely to attack NATO members, Moscow would certainly see advantages in such a development, the analyst said.

“But it really depends on where and when the troops are removed,” Lesser added, pointing out that it would be hard to estimate the effect on Europe for now. If troops are moved from Western Europe to the eastern flank, for example, that might not be concerning since countries like Germany and France are rapidly boosting their military capabilities, he said.

But the US, too, would lose out by withdrawing troops, as its ability to project power across Europe would be reduced, experts say.

Senior members of Trump’s Republican Party have voiced concern about that.

The move could send “the wrong signal to [Russian President] Vladimir Putin,” Senators Roger Wicker and Representative Mike Rogers said in a joint statement after the troops from Germany started departing.

Why are relations souring between the US and Europe?

Since his first term, Trump has repeatedly accused European NATO members of forcing the US to carry most of the financial burden of the alliance. Last January, Trump said NATO countries should start spending 5 percent of their gross domestic product (GDP) on defence, rather than rely on US support, but many NATO members are still struggling to meet the current 2 percent target.

NATO members have already agreed to boost spending on defence to 3.5 percent of GDP by 2035.

Adding to Trump’s annoyance, however, has been the refusal of European countries to join the ongoing US-Israel war on Iran by allowing the US to use European bases. Some members, such as Spain and Italy, have actively spoken out against Washington’s war.

Another issue has been tariffs. The EU exports a large amount of its goods to the US, but Trump slapped a 15 percent tariff on European imports as part of his trade war last year.

Also in the last year, the EU and Washington faced off after Trump said the US wanted to acquire Greenland, an autonomous territory of Denmark positioned strategically in the Arctic and with untapped reserves of oil and other resources. Trump threatened European countries with additional trade tariffs if they stood in his way. He later dropped that threat and said he would not attempt to take Greenland by force.

On Friday, Trump announced that he had reached a new deal with Greenland and Denmark which would grant the US “permanent control over security and all other needs” in Greenland, suggesting it could provide the US with access to natural resources as well.

Are other US allies facing issues?

Taiwan, another traditional US defence partner, is also finding it harder to rely on Washington.

Since 1950, Taipei has depended on the US’s presence in the Pacific to deter a possible attack from China. However, Trump has made several comments that have put that relationship into question.

For example, in the run-up to the 2024 presidential elections, he said Taiwan should pay Washington for protection. He has also said Taiwan should “cool down” because the US would rather not have to go to war in the region.

As the war on Iran continues, the US has increasingly diverted military assets from the Pacific to the Iran conflict, raising concerns about Taiwan’s vulnerability. Most recently, the Trump administration redeployed its last aircraft carrier remaining in the Pacific to the Middle East to relieve the Abraham Lincoln, which had been at sea for months without a break.

The US has also appeared unwilling to support Saudi Arabia, with which it has security agreements, against Yemen’s Houthi militia, while US allies in the Gulf have all sustained severe damage to energy and military facilities from Iranian strikes against US assets.