The basic law dictates compulsory military service, but that has been suspended for a while now, because after the fall of the Soviet Union the need for that went down, the Bundeswehr was of little relevance.

The basic law also grants you the right to refuse the service for conscientious reasons (like not willing to kill, religion). Usually in those cases you‘d end up doing some substitute civilian service, anyway, if otherwise you‘d be fit for military service.

What‘s happening now is, that Germany is gearing up for war against Russia. Militarization is in full swing, money is funneled towards military spending, as well as infrastructure spending that benefits the military (west-east axis to transfer equipment and units), all while spending on social, cultural and educational sectors is being cut.

And with that the state has a renewed interest in human military stock that would be available, if that war actually happened.

So the suspension is cancelled and slowly, but surely, the younger generation will be compelled to join the compulsory military service. And you‘re right, for changes in the basic law, you would need a two thirds majority.

Now here comes the „conscientious reasons“ into play, because the threshold of what counts as such is defined through normal laws and these can be changed more easily (simple majority, I believe).

Basic law defines: „you can refuse for conscientious reasons“; normal law defines: „what actually is a conscientious reason in case of refusal“.

So the problem here is, that citizens are granted the right to refuse, but the state dictates the conditions on what counts and what not.

In practice the state will create the conditions to get the amount of people into the military, they think they need. Against the will of the individual, if need be.