Video report by ITV News Correspondent Rachel Younger
Jeremy Corbyn has denied he is a "pacifist", saying he accepts military action is necessary as a "last resort".
Unveiling Labour's national security pledges on Friday, Mr Corbyn said the "war on terror" and "bomb first, talk later" approach of recent times had failed.
The Labour leader also said he would not be afraid to stand up to Donald Trump, adding that "pandering" to the US administration would not bring stability in a swipe at Theresa May.
Known as a lifelong supporter of nuclear disarmament, Mr Corbyn said he would do "everything to protect the security and safety of our people and our country" if elected prime minister.
Corbyn on military action
Mr Corbyn, who has a long record of outspoken criticism of British and US military interventions, including in Syria, Iraq and Afghanistan, said the UK's interests are best served by pursuing peace.
But insisted he is "not a pacifist", adding: "I accept that military action, under international law and as a genuine last resort, is in some circumstances necessary.
"But that is very far from the kind of unilateral wars and interventions that have almost become routine in recent times."
The Labour leader criticised the US-led "war on terror" which followed the 9/11 attacks of 2001, saying it had failed to deliver security at home or abroad.
"We need to step back and have some fresh thinking," Mr Corbyn said.
Corbyn on Trump
Speaking at the Chatham House international affairs think tank in London, Mr Corbyn accused Mrs May of "pandering" to Mr Trump, saying this would not deliver stability.
"Waiting to see which way the wind blows in Washington isn't strong leadership," he said, adding that there would be "no more hand-holding with Donald Trump".
Mr Corbyn said he wanted a "strong and friendly" relationship with the US but that a Labour government would not be afraid to "speak our mind".
And he criticised the US president's approach to confrontations with North Korea and in Syria.
He said: "The new United States president seems, sadly, determined to add to the dangers by recklessly escalating the confrontation with North Korea, unilaterally launching missile strikes on Syria and opposing what was a great achievement - President Obama’s nuclear arms deal with Iran - and the suggestion he’s backing a new nuclear arms race."
Defence Secretary Sir Michael Fallon said Mr Corbyn had shown he would put Britain's security at risk.
Sir Michael said: "He says he would never use Trident, dithers over confronting Isis (Islamic State), and wants to tie the hands of our intelligence services.
"Jeremy Corbyn is simply too weak and shambolic to stand up to terrorists and tyrants who want to do us harm."