Liz Kendall says Labour should back 2% defence spending
Labour should commit to the Nato target of spending at least 2% of national income on defence, leadership contender Liz Kendall has said.
Before the election neither the government nor Labour committed to meeting the target.
The shadow care minister also said a "new era" was needed for Labour after its "epic" defeat at the election.
She said the party must reform public services and embrace the government's education reforms in England.
Ms Kendall is one of four contenders in the race to be the next leader of the Labour Party, alongside Yvette Cooper, Andy Burnham and Mary Creagh.
The winner will be announced on 12 September, before the party's annual conference.
In a speech to journalists in Westminster, Ms Kendall said there had been "a quiet diminishing of Britain's role in the world" under the coalition government.
And Labour, she said, did "too little to challenge because we have been paralysed by the past".
But Ms Kendall pledged: "Under my leadership, Labour will no longer stand by while the prime minister weakens our country and allows the world to become less secure.
"That means insisting that the UK maintains our basic Nato commitment to continue spending 2% on defence."
The government has come under repeated pressure - from MPs and retired generals - to commit to meeting the target.
Defence Minister Philip Dunne has said a post-election spending review would set defence expenditure after 2016.
By BBC Newsnight political editor Laura Kuenssberg
It's traditional for politicians to venture into the overcrowded, stuffy Moncrieff's cafeteria in the Houses of Parliament to talk to lobby journalists who have the pleasure of working out of Westminster from time to time.
They rarely go without the intention of dropping some hint, or giving away a little bit of their thinking. But normally, just a little.
Today though, it seems Liz Kendall, one of the contenders for the Labour leadership, used the lunch to go all or bust.
It looks like she will be the only so-called moderniser in the contest.
But if being a "moderniser" means she is a new face, compared with the well-known characters of Burnham and Cooper, who both sat around the cabinet table for years, that is certainly true.
And today she made no bones about her differences.
Ms Kendall also told reporters she would not challenge the establishment of free schools and expansion of academies under the Conservatives.
"As leader I'm not going to waste time obsessing about school structures. If a school is providing a great education - whether it is a local authority, academy or free school - we will back it.
"What's more, if someone wants to help run their school, they deserve credit not criticism," she said.
She revealed that her focus would be on children's early years "not cutting university tuition fees" - a policy pursued by Ed Miliband.
Ms Kendall's speech also offered a frank assessment of the Labour Party's chances of winning in 2020.
In a withering critique of Labour's direction under Mr Miliband, the Leicester West MP said Labour had lost the election because of its policies, not his personality.
And she said the party faced being wiped out unless it could offer voters hope for the future.
"If we don't face up to what needs to be done to win back UKIP, Tory and SNP voters in England, Scotland and Wales, we will not win again.
"I think that the reasons why we lost aren't complicated, they are simple.
"We decided that the British public had shifted to the left because we wished it to happen," she said.
Ms Kendall added that the party did not have answers to the "big questions" being asked about jobs, immigration and the public finances.
The shadow minister said she wanted to lead a Labour Party that was "genuinely as passionate about wealth creation as we are about wealth distribution".
And she warned that things could be more bleak for the party in future, unless change takes place.
"We have seen this catastrophic defeat, but things could get worse for us - with a boundary review, UKIP building where they are in second place, the Liberal Democrats can only go up.
"This isn't the worst it can get. There is no God-given right for the Labour Party to exist. That is a huge challenge," Ms Kendall told reporters.