The "war on terror" has been a failure, Jeremy Corbyn said as he insisted as prime minister he would not be afraid to stand up to Donald Trump's United States administration.
"That is our first duty and to achieve it I know we would have to work with other countries to solve problems, diffuse tensions and build collective security".
"I am not a pacifist", he said.
Corbyn added that he was prepared to negotiate with ISIS, and accepts that "military action under global law is a genuine last resort [and] is in some circumstances necessary". "And pandering to an erratic administration will not deliver stability", he said.
Asked to confirm whether that would mean like-for-like replacement of the four nuclear-armed submarines required to maintain a continuous at-sea deterrent, he said: "That is the decision we will inherit as a Labour government and that is what the position is".
"The crucial thing is that there is a sharp distinction between a Government that is willing to stand up for this country, which is willing to make sure this country is properly defended and a Labour Party led by Jeremy Corbyn that would simply chuck away our ability to defend ourselves".
Jeremy Corbyn will create a Minister for Peace if he comes to power, as he warned that the country's defence policy required "fresh thinking".
"But that is very far from the kind of unilateral wars and interventions that have nearly become routine in recent times".
"A Labour Government will conduct a robust and independent foreign policy - made in Britain", he said.
He said he supported military action "as a genuine last resort" but accused recent British and US governments of "bomb first, talk later" policies.
In his speech, Corbyn - who strongly opposed the 2003 invasion of Iraq - hit out at critics claiming that he would be a weak prime minister and denied that he is a pacifist.
" If circumstances arose where that was a real option, it would represent complete and cataclysmic failure".
Unsurprisingly, the Tories say that it all shows Mr Corbyn can not be trusted with the nation's defences - accusing him of undermining Trident and dithering over IS while tying the hands of Britain's intelligence services. "It would mean world leaders had already triggered a spiral of catastrophe for humankind", he said.
However, Ian Lavery, Labour's national campaign chairman and most recent MP for Wansbeck, said: "This is nonsense".
He will say a "bomb first, talk later" approach to security "has failed" and is a "recipe for increasing, not reducing threats and insecurity".
Mr Corbyn said Labour would retain the commitment to the North Atlantic Treaty Organisation target of spending 2% of GDP on defence, and strongly criticised the Conservatives' record on the armed forces.
"I will be reaching out to all those who have been abandoned by Labour and let down by government for too long".