Fears are mounting Donald Trump could order U.S. troops out of Ukraine amid a looming spending row at a crunch Nato summit this week.
The U.S. President is expected to use the gathering in Brussels to ramp up his demands for Nato allies to take on more of the burden.
The row will continue into Mr Trump's first visit to Britain as President at the end of the week.
After spending four days in Britain, Mr Trump will fly on to talks with Russian President Vladimir Putin - raising concern he will make new agreements with the Kremlin just days after rowing with European allies.
America has thousands of troops on Europe's eastern frontier with Russia and a pull out would dramatically strain UK and European forces who take part.
Ahead of the visit, Theresa May told the Sunday Times today defence would be a key part of her talks with the President.
She said: 'I think there's going to be a key focus this week on our relationship on security and defence.
'I'm going to be having discussions with the president on a number of issues.
'The importance of the unity that we show in relation to Western security and defence I think will be part of our discussions.'
The UK, along with the U.S., meets its commitment to spend 2% of GDP on defence but the failure by other Nato members, including Germany, to meet the target has angered Mr Trump.
The Prime Minister said: 'We're meeting the pledge. A limited number of other Nato countries are meeting it.
'I would urge everybody to work towards that.'
Mr Trump's schedule will largely keep him out of central London and it seems unlikely that the president will come close enough to Westminster to see a 'Trump baby' blimp which protesters plan to fly over the Palace of Westminster, after receiving permission for the stunt from mayor Sadiq Khan.
Mr Trump arrives in the UK on board Air Force One on Thursday afternoon, straight from the Nato summit in Brussels and will carry out a series of engagements on Friday before heading to Scotland for the weekend.
Mrs May said her husband Philip was looking forward to meeting Mr Trump's wife Melania, the two spouses will attend events together on Friday.
The Prime Minister said: 'He's looking forward to meeting Melania.
'They were both at the G20 but because of timetables and so forth he wasn't able to meet her there.
'He has been out and bought a new suit.'
The president will travel to Helsinki to meet Russian leader Vladimir Putin on July 16, meaning the Nato summit and the UK visit are crucial moments in trying to persuade the U.S. leader to remain committed to Nato.
Defence minister Tobias Ellwood told the Sunday Telegraph: 'Given the U.S.'s footprint in Europe and Putin's recent hostile actions ... much hinges on the outcome of the two forthcoming summits.'
Despite the planned protests and the controversy surrounding the president, the UK's ambassador to the U.S. said he will be an 'honoured guest'.
Sir Kim Darroch has said the Anglo-American alliance plays a 'leading role'.
Writing in the Sunday Telegraph, Sir Kim said the relationship between the United States and United Kingdom was 'interwoven at every level', adding he was grateful for American support on a free trade agreement post-Brexit.
He wrote: 'President Trump will be an honoured guest, representing a partnership that is greater and deeper than any other in history.
'And in a world in which the challenges seem ever more daunting, I'm confident that the UK-U.S. alliance will continue to play a leading role in advancing our shared values of democracy and freedom.'
Meanwhile, Sir Kim's American counterpart, Ambassador Woody Johnson, said Mr Trump was trying to get as much impact as possible from his stay in the UK on what he described as a 'very important' visit.
This article has been adapted from its original source.
© Associated Newspapers Ltd.