National service would engage young people in society again, Home Secretary James Cleverly has told the OceanNewsUK, adding "too many young people live in their own bubble".
The Conservatives have set out a pledge for all 18-year-olds to take part in a mandatory scheme for military or non-military service if they win the general election.
Mr Cleverly said it would "address the fragmentation in society" - but added that sending people to jail for not taking part was not an option.
Labour said said the plan was "a desperate gimmick" with no viable funding.
Mr Cleverly told the OceanNewsUK's Sunday with Laura Kuenssberg programme the move was about "coming together".
"It's about addressing the fragmentation that we have seen in society," he said.
"Too many young people are living in their own bubble, whether that's a digital bubble or a social bubble.
"We want to get back to a situation where young people are mixing with people - in different areas, different economic groups, different religions - to try and find a way of addressing the kind of fragmentation that we see too much of."
The Conservatives say the scheme would involve 30,000 selective military placements where "the brightest and best" would get involved in cyber security, logistics, or civil response operations full-time for a year.
Everyone else would do 25 days, or one weekend a month for a year, with non-military organisations including the fire service, the police, the NHS or charities.
The armed forces placements were "a small element" of the plan, Mr Cleverly said, as "nobody will be compelled to do the military bit".
"There's going to be no criminal sanction - There's no one going to jail over this," he told Sky News.
Mr Cleverly insisted the plan was "fully funded", with £1.5bn diverted from levelling up's UK Shared Prosperity Fund from 2028. A further £1bn would come from plans to crack down on tax avoidance and evasion.
Appearing on the same programme, Labour's shadow chancellor Rachel Reeves said: "This is just another gimmick, a desperate gimmick from the Conservative Party with no viable means of funding it.
"One minute they say levelling up is really important, then they raid the levelling up budget and say it's going to be used for national service.
"This is just another example, I'm afraid of a gimmick, where the sums don't add up."
Reform UK's honorary president Nigel Farage labelled the plan "a joke" and "totally impractical".
He accused the Conservatives of making policy based on "a focus group of half a dozen Reform voters" who supported national service.
"When you're a weak leader - and Sunak is not a leader in any way at all - you're a follower, so you follow what the focus groups say, and you say 'by doing this I can attack the Reform vote'. That's what it's all about.
"Look, it's totally impractical - the Army has shrunk from 100,000 to 75,000 in 14 years of conservatism and, most interestingly, we have a growing number of young people in this country who do not subscribe to British values, in fact loathe much of what we stand for." he said.
The Liberal Democrats said Mr Cleverly's Braintree constituency had benefited from £1.6m from the UK Shared Prosperity Fund and accused him of putting people there "high and dry... in a desperate plea for headlines".
The party's housing and local government spokesperson Helen Morgan said the Conservatives should instead focus on reversing a 10,000 cut in Army troop numbers.
SNP deputy leader Keith Brown said the plan was only "half-thought through" and proper investment was needed to boosting recruitment to the armed forces.
Mr Brown, a former Royal Marine, said: "The bubble they're in is one of a lack of investment in public services which support them and the opportunities which flow from that and that's entirely down to the Conservative Party.
"They're trying to put a sticking plaster on the future of young people and the future of the armed forces and it's not going to work."
The Scottish Conservatives welcomed the initiative, noting that Scandinavian countries had similar systems.
But Green Party co-lLeader, Carla Denyer said the policy was "removed from reality".
"It's not what our military needs and it certainly isn’t what our young people need.
"What young people tell us they need is access to the housing market, to higher education that doesn’t plunge them into debt, and to meaningful jobs that pay well - not military conscription," she said.
