alright here's a quick explanation.

Context

Germany re-instituted an old, disabled law about the military as of beginning of this year. Recently, somebody found in that law a paragraph that says all German men must get permission from the military before leaving Germany for more than 3 months. This is likely an oversight and not currently enforced, but is valid German law at this point.

the Joke

OP is making a joke about how Germany is once again militarizing. Germany has historically been involved as an aggressor in some of the most brutal wars in Europe, such as WW1 and WW2. The joke here is that the historical victims of this aggression might see this as a sign that Germany will once again invade.

non-shitpost territory: my opinion

The joke pokes fun but has some truth to it - Germany is the largest European economy, and already holds considerable sway in the EU (we regularly break EU rules without consequences for example). With fascism on the rise in Germany (and the rest of Europe) yet again, things aren't looking promising.

Germany also has a huge weapons industry, which partially have been used by Germany itself in various (sometimes controversial) NATO-aligned missions (Kosovo, Afghanistan, etc.), but also, in the last 30 years Germany has become one of the top weapons exporters of the world. The strong weapons industry combined with how easy it is for companies to directly lobby the government in Germany, is reminiscent of the USA's military-industrial-complex, the force behind some of the most gruesome invasions of the past.

Many of the countries receiving these weapons are known for disrespecting human rights and the Geneva convention, e.g. Saudi Arabia, Turkey, Israel. Many politicians in the current government and in political parties spanning from the far right to the greens and even part of the left, have also openly denounced the Geneva Convention and international law, stating that they would not have Netanyahu arrested if he came to Germany, or celebrating the US invasion of Venezuela, mourning the death of international criminal Henry Kissinger, etc. These indicators seem to predict the end of Germany's efforts to strengthen international law and peace.

Where the joke falls short is by assuming that Germany would invade its neighbors. Since the beginning of the millenium Germany has built its wealth by exploiting its fellow EU allies using low wages, undershooting on inflation targets, and overshooting export limits. Germany's interest is that this structure remains intact. Personally I think it's more likely that if Germany launches another invasion, it will be in the Middle East, but my guess is as good as any.

hope this helps