Jeremy Corbyn has highlighted the link between the UK's involvement in foreign wars and terrorism at home but insisted that "in no way reduces the guilt" of murderers who have targeted people on Britain's streets.
In a speech marking the return to General Election campaigning following the Manchester atrocity, Mr Corbyn said the "war on terror has not worked" and vowed to tackle the causes of terrorism.
The Labour leader said the presence of troops on Britain's streets was an indication the current approach was not working as he vowed to be tough on the causes of terrorism.
Attacking Conservative austerity measures under Theresa May, first as home secretary and then as Prime Minister, Mr Corbyn vowed to increase funding available for the police and emergency services.
The speech in London was preceded by a minute's silence for the 22 people who lost their lives in Monday night's attack at the Manchester Arena.
Mr Corbyn was criticised by political rivals from the Tories and Liberal Democrats over the content and timing of his speech while the investigation into the attack was ongoing.
The Labour leader said: "Many experts, including professionals in our intelligence and security services have pointed to the connections between wars we have been involved in or supported and and fought in other countries, such as Libya, and terrorism here at home.
"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.
"But 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.
"Protecting this country requires us to be both strong against terrorism and strong against the causes of terrorism.
"The blame is with the terrorists, but if we are to protect our people we must be honest about what threatens our security.
"Those causes certainly cannot be reduced to foreign policy decisions alone.
"Over the past 15 years or so, a sub-culture of often suicidal violence has developed amongst a tiny minority of, mainly young, men, falsely drawing authority from Islamic beliefs and often nurtured in a prison system in urgent need of resources and reform.
"And no rationale based on the actions of any government can remotely excuse, or even adequately explain, outrages like this week's massacre.
"But we must be brave enough to admit the war on terror is not working.
"We need a smarter way to reduce the threat from countries that nurture terrorists and generate terrorism."