Labour leader Jeremy Corbyn has suggested the UK should not automatically come to the aid of Nato allies facing aggression.
Speaking at last night's Labour leadership hustings against Owen Smith, Corbyn said he will seek to use diplomacy, rather than use armed force.
Republican presidential candidate Donald Trump has made similar suggestions on the US' Nato obligations.
Read More: Trump and Corbyn are pushing extreme views
The alliance enshrines principles of “collective defence”, where an attack on one member is considered an attack on all. The Labour contenders were asked how they would react if a Nato ally faced aggression from Russia, in particular.
"I would want to avoid us getting involved militarily by building up the diplomatic relationships,” Corbyn said.
Pressed to clarify, the Labour leader added added: "I don't wish to go to war. What I want to do is achieve a world where we don't need to go to war, where there is no need for it. That can be done."
By contrast, Smith was clear-cut: “We would have to come to the aid of a fellow member of Nato. That would be the job of Britain in the event of a fellow Nato member being invaded, obviously" - while stressing it would be "calamitous and we must never see it happen".
Foreign policy has proved troubling ground for both Labour's leadership contenders – earlier this week Smith was forced to clarify his views after suggesting he will negotiate with terror group Islamic State in a bid to secure peace in the Middle East.
Writing on Facebook, the Pontypridd MP said: "There can be absolutely no negotiation with any terrorist group until they renounce violence, cease all acts of terror and commit themselves to a peaceful settlement.
"My experience of working on the peace process in Northern Ireland, though, is that eventually all parties who truly believe in delivering peace have to be around the table. In the Middle East at the moment that clearly doesn't include - and may never include - Isis."