NATO's former Deputy Supreme Allied Commander Europe, Richard Shirreff, has warned that some of Donald Trump's foreign policy statements could mean the beginning of the end of the alliance.
Speaking on the BBC's Newsnight at the end of last week, Mr Shirreff addressed the issue of Donald Trump's remarks on campaign that he would not commit the US to defending another NATO member if they are not contributing 2% of GDP to defence.
The former NATO deputy said that President-elect Trump will need to make it "crystal clear" once inaugurated that he will come to the defence of a fellow NATO member, "no ifs, no buts, no prevarication":
"If that doesn't happen, there will continue to be doubts about America's willingness to [help defend fellow NATO members] under President Trump, and that will strike right at the heart of NATO's founding principle of collective defence".
The Article 5 commitment to mutual self-defence by all NATO members is, Mr Shirreff says, the foundation of NATO and a failure to remain committed to it by America could mean the beginning of the end for the alliance.
Lt Gen Sir Richard Shirreff at Chatham House (image: Chatham House)
The US is currently one of only five members who meet the required funding target (the others are Greece, Poland, Estonia, and the UK).
More: NATO Secretary General says he has confidence America will continue to support the alliance on 'Sitrep, November 24th 2016'
The aim is for all 28 members to have reached the 2% target by 2024.
Five members - Canada, Spain, Belgium, Luxembourg, and Slovenia - are currently spending below 1% of GDP, however, so it may prove difficult for them to reach this.
Additionally, Newsnight presenter Mark Urban points out that, if Mr Trump tries to improve US relations with Vladimir Putin by recognising the Russian annexation of Crimea or "allowing free reign within the former Soviet Union", this could cause even greater tension with NATO members.
This is because three former member states of the USSR are now in NATO: Estonia, Latvia, and Lithuania. Georgia is in the process of discussions about becoming a member. And Ukraine has moved politically in recent years between being pro-European and pro-Russian.
The country voted overwhelmingly to be independent from Russia when the USSR broke up in 1991.
In more recent years, relations between the two countries have been tense. Ukraine began a diplomatic relationship with NATO in 1994, and applied for NATO's MAP (Membership Action Plan) in 2008 under President Viktor Yushchenko.
Yushchenko was officially defeated by Viktor Yanukovych in a controversial election in 2004 which led to the 'Orange Revolution'. Wide-scale protests took place in response to electoral fraud and an attempt on Yushchenko's life (he was poisoned and disfigured, although he survived and made a full physical recovery).
In a run-off vote Yuchchenko won and became president in 2005, though he later lost popularity and only served until 2010. Yanukovych won the presidency, legitimately this time.
Viktor Yushchenko, president of Ukraine from 2005 - 2010 (left; image: Erfah Kouchari); Viktor Yanukovych, president from 2010 - 2014 (middle; image: Premier.gov.ru), and Petro Porochenko, current President of Ukraine (right; image: Photo Claude TRUONG-NGOC)
Once in power, he reversed Ukraine's bid for NATO membership, rejected an EU trade deal, and sought a Russian bailout. More protesters took to the streets of the capital, Kiev.
When they were violently suppressed, Yonukovych was removed from power by the government and fled to Russia.
Petro Poroshenko, a supporter of the protests, became the new President in May, 2014.
A map of Ukraine with Crimea shown in orange (image: Spiridon Ion Cepleanu)
Ukraine's internal dispute about the direction of its future, either west towards the EU, or east towards Russia, hinges on the fact that many people in the east of the country are Russian-speaking and identify as Russian culturally, while most people in the west do not.
NATO itself also recognises that Russia sees increasing NATO membership with states on its border as an isolating move.
In response to Poroshenko's election (and Ukraine's anticipated tilt back towards Europe), pro-Russian protesters in Crimea called for the country to move in the other direction.
Russia moved troops into Crimea, in southern Ukraine. Politicians in Kiev called this an 'invasion', though Crimean politicians called for a referendum and the result was that 90% of the population chose for Crimea to be part of Russia instead of Ukraine. NATO called it an illegitimate result, given the context of Russian troops moving into the territory and the possibility of a flawed election.
Tensions have been particularly strained between NATO members and Russia ever since, soured further by the apparently accidental shooting down of airliner MH-17 by Russian-backed rebels in eastern Ukraine in July, 2014.
Former NATO Deputy Secretary-General Alexander Vershbow
Addressing the issue of Trump's call for improved relations with Russia on Newsnight, Alexander Vershbow, a former Deputy Secretary-General of NATO, compared the situation to the 1945 Yalta Conference.
It saw Russia, Britain, and the US agree to prioritise the unconditional surrender of Nazi Germany and to divide Germany into zones administered by the three powers and France when the Second World War ended. They also agreed at Yalta (in the Crimea) to allow the European states to select governments of their own choice through democratic elections.
The Yalta Conference of 1945, which would help shape the future of post-World War Two Europe; Churchill is seated left, American president Franklin Roosevelt middle, and Stalin right
But Stalin pressured countries in eastern Europe to form communist governments. Ukraine at this time had also been absorbed into the USSR, despite declaring independence in 1917. In the subsequent Soviet famine of 1932-33, many of the millions who died were in Ukraine.
Looking forward, Mr Vershbow put it that:
"Ratifying the results of Russian aggression in Ukraine would buy you some short-term tranquility, but in the end it would create a much more unstable situation in Europe, [and] encourage the Russians to continue to press forward for some kind of Yalta Two, with a new division of Europe into spheres of influence, which I think would bring back some of the instability that we saw in previous decades."
Ultimately, time will tell how the world's changing political landscape will affect the NATO alliance, Europe, Russia and the Ukraine, with the results to become apparent in the coming years.
Cover image: Gage Skidmore