The Military Service Modernisation Act sets out plans to expand the number of active personnel from around 180,000 to 260,000 by 2035.
In December, the German parliament voted to introduce voluntary military service, meaning that from January all 18-year-olds would be sent a questionnaire asking if they were interested in joining the armed forces.
From July 2027, they must also undergo a fitness assessment to determine whether they would be eligible for service should war break out.
Women may volunteer for military service but cannot be compelled to serve under Germany’s constitution.
While the plan is for voluntary service, if the security situation worsens or if too few volunteers came forward, a form of compulsory military service could be considered.
When the law was approved by parliament, many young people joined protests against the change.
“We don’t want to spend half a year of our lives locked up in barracks, being trained in drill and obedience and learning to kill,” one organiser wrote on social media.
Like other European countries, Germany ran down its armed forces during the peacetime years of the 1990s. During the Cold War it had an army of almost half a million.
Compulsory military service in Germany was ended in 2011 under then-chancellor Angela Merkel.
Chancellor Friedrich Merz has pledged to rebuild the Bundeswehr into Europe’s strongest conventional army in response to what his government describes as a more dangerous security environment in Europe.