Jeremy Corbyn declared he is "no pacifist" as he looked to downplay Conservative claims that he is weak on defence.
The Labour leader has been an outspoken critic of military intervention throughout his career, but said that he accepts conflict is needed "as a last resort" at a speech in London on Friday (12 May).
Corbyn also launched his own attack on the Conservatives, declaring that the "bomb first, talk later" approach to foreign policy has failed and has made Britain more insecure.
It comes as the UK is involved in several covert wars with engagements in Syria, Iraq, Libya, Yemen and Afghanistan, among others.
Speaking from the Chatham House think tank, Corbyn said: "They [the Conservatives] have not increased our security at home – rather the opposite – and they have caused destabilisation and devastation abroad."
He added that "waiting to see which way the wind blows in Washington" is not strong leadership and that "pandering to an erratic Trump administration will not deliver stability".
Corbyn also said: "Britain deserves better than simply outsourcing our country's security and prosperity to the whims of the Trump White House.
"So no more hand-holding with Donald Trump – a Labour government will conduct a robust and independent foreign policy made in London.
"This is the fourth general election in a row to be held while Britain is at war and our armed forces are in action in the Middle East and beyond.
"The 'bomb first, talk later' approach to security has failed. To persist with it, as the Conservative government has made clear it is determined to do, is a recipe for increasing, not reducing, threats and insecurity."
In a statement, the Conservatives dismissed Corbyn's speech as "nonsense" and claimed that he has "spent a lifetime trying to disarm Britain".
"Only Theresa May and the Conservatives will provide the strong and stable leadership needed to defend our values against those who seek to destroy our way of life," the statement added.