UK foreign policy would change under a Labour government to one that "reduces rather than increases the threat" to the country, Jeremy Corbyn has said.
He said "many experts" had linked UK involvement in wars abroad to terrorism at home, as election campaigning resumed after the Manchester attack.
He also pledged to reverse police spending cuts - but stressed the blame for attacks lay with the terrorists.
Opponents criticised his speech, coming days after the attack which killed 22.
Foreign Secretary Boris Johnson said it was "absolutely monstrous" that the Labour leader had attempted to "justify" or legitimise the actions of terrorists.
And Lib Dem Leader Tim Farron accused Mr Corbyn of using the "grotesque" attack in Manchester to "make a political point".
Mr Corbyn delivered his speech as the bigger political parties return to the campaign trail following Monday night's suicide bombing at Manchester Arena.
He pledged a "change at home and change abroad" if Labour won power - with a new foreign policy approach and more money for police, the NHS and security services.
He said he did not want to make a "narrow political point" as Britons had to stand together after the attack.
But he added: "An informed understanding of the causes of terrorism is an essential part of an effective response that will protect the security of our people, that fights rather than fuels terrorism."
He said that "many experts, including professionals in our intelligence and security services, have pointed out the connections between wars that we have been involved in, or supported, or fought, in other countries and terrorism here at home".
But he added: "That assessment in no way reduces the guilt of those who attack our children.
"Those terrorists will forever be reviled and implacably held to account for their actions.
"The blame is with the terrorists, but if we are to protect our people we must be honest about what threatens our security."
He said the causes "certainly cannot be reduced to foreign policy decisions alone" and said the prison system, where a "subculture of often suicidal violence" had developed among some young men, was also in need of "resources and reform".
Mr Corbyn also said that a Labour government would fund more police and ensure the security services had sufficient resources "to keep track of those who wish to murder and maim".
On deploying British troops, he said he would tell them: "You will only be deployed abroad when there is a clear need and only when there is a plan that you have the resources to do your job and secure an outcome that delivers lasting peace".
'Extraordinary and inexplicable'
Former MI5 chief Baroness Manningham-Buller told the Iraq Inquiry in 2010 that the 2003 invasion of Iraq had "undoubtedly increased" the terror threat to the UK and had radicalised "a few among a generation who saw our involvement in Iraq, on top of our involvement in Afghanistan, as being an attack on Islam".
Speaking in London at a press conference with US Secretary of State Rex Tillerson, Boris Johnson said: "Now is not the time to do anything to subtract from the fundamental responsibility of those individuals, that individual in particular, who committed this atrocity.
"And I think it is absolutely monstrous that anybody should seek to do so.
"I have to say I find it absolutely extraordinary and inexplicable in this week of all weeks that there should be any attempt to justify or to legitimate the actions of terrorists in this way."
Defence Secretary Sir Michael Fallon said the way to deal with terrorism was about being "tough at home" with anti-terror laws and powers for police and security forces, while taking military action "when you have attacks being planned against this country".
He added: "It is a fact of record that Jeremy Corbyn has opposed every serious piece of anti-terrorism legislation throughout his time in Parliament."
Lib Dem Leader Tim Farron said he did not agree with what Mr Corbyn said, "but I disagree even more that now is the time to say it.
"That's not leadership, it's putting politics before people at a time of tragedy."
SNP leader Nicola Sturgeon said the only people to blame for the Manchester terror attack were "the man who carried it out and anybody else who aided and abetted him".
"But we must have a proper debate about foreign policy and whether that is helping or hindering the challenges, including terrorism, that the world faces today."
Paul Nuttall, leader of UKIP, said he "tended to agree" with Mr Corbyn on the link between British foreign policy and an increased UK terror threat.
"If the Tories got their way we would have ended up in Syria now, and it would have been even worse.
"However, I hope Jeremy Corbyn isn't using this as an excuse for what happened the other night."