United Kingdom

All 18-year-olds will do this new National Service, whatever their background and wherever they live in the UK. It will not, though, be a one-size-fits-all programme. Young people will be able to choose between 25 days of volunteering – equivalent to one weekend every month – serving as a lifeguard for the RNLI, becoming a first aider or many other options.

Those who choose to do military service, and pass the test, will be able to take a 12-month, full-time placement in our Armed Forces.
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To those who complain that making it mandatory is unreasonable, I say: citizenship brings with it obligations as well as rights. Being British is about more than just the queue you join at passport control.

https://www.dailymail.co.uk/news/article-13460055/prime-minister-rishi-sunak-plan-happy-daughters-national-service.html

Talking to Sky News's Sunday Morning With Trevor Phillips, Home Secretary James Cleverly said: "There's going to be no criminal sanctions, nobody's going to jail over this."

https://news.sky.com/story/refusing-mandatory-national-service-wont-lead-to-prison-home-secretary-says-after-tory-policy-launch-13143272