If you believed everything you read in the Daily Mail, a former Royal Navy Commodore is not the sort of person you would expect to be campaigning rigorously to put Jeremy Corbyn in Downing Street. But Gareth Derrick, Labour's prospective parliamentary candidate for South East Cornwall, is doing exactly that.

"There's a real anger on the doors against politicians in general," says Derrick, who stood in the same constituency in 2017 and is also a councillor in Plymouth. "But there's also an understanding that the country is heading in a dangerous direction, and we have to do something about it."

In the aftermath of Friday's London Bridge attack, and as NATO leaders gather in London for their summit, the Conservatives reportedly want to make "security" one of their major buzzwords this week, hoping to capitalise on what they hope is Labour weakness on the issue. But that might be harder than they hope.

Derrick is one of at least five former military candidates standing for the Labour Party this election, and the most senior officer ever to stand for the party. Others include Sarah Church, a former Royal Signals officer standing in South Swindon, and former RAF man Tom Corry, standing against Jeremy Hunt in South West Surrey. They reflect a wider trend – that beyond the headlines, Labour has been quietly notching up its credibility on national security and military affairs, with the Conservatives vulnerable on what they once saw as a bedrock strength.

This isn't just about resources – although as with every other public service, this government has both slashed spending on the military, diplomacy and aid, and outsourced to contractors who have often been incompetent. It also speaks to mounting disquiet about the way in which Boris Johnson's party is embracing the playbook of Donald Trump and Vladimir Putin, including the use of outright lies as an integral part of both domestic politics and international diplomacy.

That makes this week's NATO summit particularly dangerous territory for the Conservatives – particularly if Trump chooses to play to his US electoral base by picking fights with other attendees. Some of Britain's closest allies, frankly, would be rather more comfortable with the "new internationalism" of Labour's manifesto pledges than Johnson – who I've heard described on the continent as "Britain's Berlusconi" – and the pro-Brexit forces that surround him.

Such concerns have been deepened further by the refusal of the government to release the Parliamentary Intelligence and Security Committee report into Russian meddling in British politics. Following Friday's attack, it took only a matter of hours – perhaps even less – for false messaging to begin spreading on whatsapp. An Australian reporter was shown a fake tweet by a taxi driver supposedly from Jeremy Corbyn attacking the police for shooting the attacker dead. In reality, Corbyn was swiftly clear he felt the police had acted correctly.

At a debate on defence last week at London's Royal United Services Institute, Defence Secretary Ben Wallace was clearly under instructions to turn the conversation to Corbyn at every opportunity. But those on the floor asked at least equally tough questions over the prime minister and his record – while the wider discussion also showcased the very solid offer on wider defence the party has pulled together under Shadow Defence Secretary Nia Griffith.

There is plenty that needs fixing. Under the Tories, trained army strength has fallen from 102,000 to just under 74,000, with a botched Capita recruitment contract coming on top of a much wider retention crisis. The RAF lacks enough pilots, while the Navy has shrunk dramatically and struggles to get even the it does have to sea. The Labour Party is committed to spending a true two percent of GDP on defence – the most recent Conservative government has fudged the numbers by including things like military pensions – as well as making sure Armed Forces personnel get a proper pay rise after seven years of below inflation increases.

Quietly, the Labour Party is making inroads amongst forces communities. I was out campaigning for the party this weekend in Aldershot, where Labour almost doubled its vote in 2017 to reduce the majority of incumbent Tory Leo Docherty to just over 10,000. Activists say those gains are holding up, even with challenges over Brexit – the local food bank reports demand in the constituency is growing by 20 per cent a year, with many of the recipients veterans and their families. Even amongst those openly skeptical of Corbyn, there is heightened interest in what the Labour Party offers.

That includes a rocksolid manifesto commitment to renewing Trident. For all the attempts to paint Corbyn as undermining its deterrent effect through his reluctance to use it, the decline of Jo Swinson suggests the electorate don't want someone overly enthusiastic about incinerating cities. Corbyn's thoughtfulness on use of lethal force and reluctance to use military force are often used as arguments against him, but in truth they cut both ways.

The Conservatives could yet be successful in framing this election as a decision on whether Boris or Corbyn are best trusted with Britain's national security. But they might not like the answer.

Peter Apps is a global affairs commentator for Reuters, British Army reservist and active member of the Labour Party.