In June 2014 during an interview with the LBC radio host Nick Ferrari, Boris Johnson agreed to be blasted by a water cannon. “Man or mouse. All right, you’ve challenged me to this. I suppose I’m going to have to do it now,” the then mayor of London said with trademark bombast.

True to form, it was a promise Johnson never fulfilled. The rhetorical flourish came during a heated debate over the use of water cannon as the Metropolitan police argued that high-velocity blasts of water would be a useful tool should there be any repeat of the 2011 England riots.

Opposition politicians and human rights groups argued that the weapons were inhumane, as did Dietrich Wagner, a German pensioner who was blinded by a water cannon during an environmental protest in Stuttgart. Johnson pressed on and bought three at a cost of £322,834.71. Four years later they were sold unused for £11,025 to a scrapyard in Nottinghamshire.

The saga stands out when Johnson’s record on policing is brought to mind. Now as prime minister Johnson has promised to recruit about 20,000 more police officers. His home secretary, Priti Patel, responsible for delivering his law and order vision, talked outside the Home Office after her appointment about tackling the “scourge” of crime. Johnson will hope his record on policing is rewritten.

The police recruitment drive is the central commitment and there are plans to start hiring in September. Less eye-catchingly, there will also be a national policing board, designed to hold the police to account for meeting the 20,000 target and to scrutinise forces’ responses to pressing issues. A review of stop of search powers has also been announced.

“As I said on the steps of Downing Street this week, my job as prime minister is to make our streets safer,” Johnson said in a press release formalising the commitment.

The hiring spree is an attempt to combat a high-profile surge in knife crime in England and Wales. There were 43,516 knife crime offences in the 12 months to the end of March, the highest number since comparable records began and up 80% from the lowest on record, 23,945 in the year ending March 2014.

More than 100 fatal stabbings have been recorded in Britain this year. Cases such as those of 17-year-old Jodie Chesney and 14-year-old Jaden Moodie have focused the public’s attention on what at times feels like an epidemic.

The Conservatives’ claim to be the party of law and order is under threat: violent crime is up, police numbers are down and the proportion of crimes solved has fallen by half in four years. Fewer than one in 12 offences – 7.8% – result in a charge or summons, compared with 9.1% last year and 15% four years ago.

Unsurprisingly, the police have welcome Johnson’s commitment. The National Police Chiefs’ Council chair, Martin Hewitt, said: “This substantial growth in police officers will ease the pressure on our people and help us to reduce crime and improve outcomes for victims. It is also an incredible opportunity to accelerate our plans to increase diversity in policing.”

But in truth the recruitment drive should be seen a restoration of police numbers, which have plummeted by about 20,000 to 123,000 since the Conservatives came to power in 2010. Furthermore, factoring in the number of officers who leave the service each year – about 6% of the total workforce – the target number may barely make a dent.

Then there is the question of whether rising violent crime can be tackled through increased policing alone. Theresa May, who as home secretary and prime minister maintained a much more confrontational approach to dealing with the police service, challenged the link between officer numbers and rising violent crime.

The figures certainly create some room for doubt. Violent crime as recorded by police has been increasing since 2014 but fell between 2009 and 2014 at the same time as police officer numbers were being cut. In 2008 when police numbers were at a high, knife deaths of teenagers and children were higher than they had been over the past 10 years.

But if it is accepted that shrinking police forces are relevant to rising crime, to focus solely on officer numbers would still be to ignore a wide range of underlying issues. All of those factors can also be linked to cuts in public spending.

The government’s serious violent crime strategy states that like other types of crime and antisocial behaviour, violent crime has a clear link to “poor life outcomes”. Low educational attainment, poor health including mental health, unemployment, socio-economic factors, weak ties to family and exclusion rates all have links to violent crime.

Alongside policing budgets, funding is being cut for early intervention services that could go some way to tackling some of the related issues. A report, Turning the Tide, produced by Action for Children, the Children’s Society and National Children’s Bureau, revealed that between 2010/11 and 2015/16 spending on early intervention fell in real terms by 40%. Funding of Sure Start centres, which provide access to early intervention services, halved over eight years.

Sajid Javid, toward the end of his short tenure as home secretary, was coming round to more innovative approaches to violent crime, treating the surge in violence as a health issue and placing a new public health duty on bodies in England and Wales.

Many have already questioned whether it would be more effective to invest in such approaches, including mental health support or early intervention services, rather than in bobbies on the beat.

Oft-cited Home Office research from the mid-80s concluded: “A patrolling police officer could expect to pass within 100 yards of a burglary taking place roughly once every eight years. Even then they may not even realise that the crime is taking place.” The climate is quite different but the fundamental theory behind this statistic is worth considering.

Boris Johnson says he’ll spend, but who will pay? Read more

Marian Fitzgerald, a former principal researcher in the Home Office research and statistics directorate and now visiting professor of criminology at the University of Kent, said: “Boris Johnson has shown a cavalier disregard for the police in promising to recruit 20,000 extra police officers over the next three years. This has nothing to do with what struggling forces want or need.

“The promise is calculated to appeal to the public. Yet if he keeps it, with no commitment to fund other aspects of policing, the public will be paying a lot more for little observable improvement – whether in detection rates or in the service they receive if they themselves try to call the police.”

She added: “Simply boosting police visibility will not deliver these improvements if the infrastructure which should support the work of frontline officers continues to crumble and no provision is made for essential improvements to the equipment they need to do the job.

“At worst, flooding the frontline with probationers could be a recipe for disaster in the absence of sufficient numbers of experienced officers capable of mentoring and supervising them.”

Johnson for the time being has placed all his chips on frontline policing. The crime statistics over the next few years could reveal whether the gamble pays off.