NATO allies worry that the six-month review, announced in June, will lead to large-scale cuts to the roughly 80,000 U.S. forces across Europe and punish countries that President Donald Trump believes failed to support the U.S.-Israeli war against Iran.

The Pentagon's 'Terms of Reference,' which have not been previously detailed, explain the steps that the internal review will take and some of the criteria that will be used to evaluate U.S. basing decisions throughout Europe.

One important disclosure in the document is a plan for the top U.S. general in Europe to participate in a decision briefing with the Pentagon's policy chief, Elbridge Colby, by September 18, according to the document.

That briefing will be key, according to the document, in hashing out options to develop for Hegseth.

Colby is expected to speak with NATO ambassadors about the review on Thursday at the alliance's Brussels headquarters.

Asked about the Terms of Reference, a senior Pentagon official said the fact that a spectrum of options would be generated, in detail, underscored the seriousness of the process.

'We want to be able to talk through the pros and cons of a variety of different force levels or dispositions,' the official told Reuters.

It is unclear, however, whether the details will convince European officials sceptical of the review process, who say privately they are concerned Washington's decision is all but made.

The Trump administration has long signaled it intends to reduce the military resources it devotes to Europe, with the expectation that NATO allies will carry a greater share of the burden than they have historically.

Trump has also fumed that European NATO allies have long relied on U.S. security guarantees while providing inadequate support for the Iran war. Accusations include that some allies failed to grant basing and overflight access to U.S. warplanes, or did it too slowly.

NATO Secretary General Mark Rutte has expressed some understanding of Trump's disappointment but said the large majority of Europeans have been helpful.

The Pentagon official said the Trump administration had made clear that 'we are transitioning to provide critical, more limited support with allies in the lead.'

'President Trump has also been quite rightly and clearly frustrated with certain allies' behaviour in recent months. So there's clearly at least some demand signal to have options that would show us (downgrading) our force levels in Europe,' the official said on condition of anonymity to discuss the matter.

Still, the official said the review sought to 'provide a spectrum of options from where we are to wherever' the review leads.

Announcing the review in June, Hegseth chided some 'free-riding' NATO allies and said the review would ensure U.S. military 'access, basing, and overflight are clearly delineated.'

The Terms of Reference go further.

Beyond calling for reliable U.S. access, basing and overflight rights, called ABO, in allied NATO nations, they examine space, cyber and 'intelligence architectures and infrastructure that enable U.S. global power projection,' the document says.

The Pentagon expanded the definition of ABO to ensure the U.S. takes into account all the types of support that NATO allies provide, the senior official said.

The Terms of Reference also address other ways allies will be evaluated, and appear to be in line with previous U.S. remarks by senior officials.

The document says the Pentagon will seek to determine whether allies have a credible path to meet their NATO spending pledges and fill in gaps to NATO crisis forces — known as the NATO Force Model — after U.S. reductions were announced earlier this year.

(with inputs from Reuters)