You defend yourself, your family, your relatives and your friends.

You are not strong enough to defend these by yourself, so you can do this only by joining the army of the country in such cases.

It would be great if these kinds of actions would have become unnecessary in the modern society, but wars are still started by despicable humans like Putin, Trump and their associates, and there are even hundreds of millions of people who appear to approve such actions.

In the distant past there have existed a few "civilized" wars, where for much of the population it did not matter who won the war, because that just meant that they would pay the same taxes as before to a different authority, but they would just go on with their lives.

However, this has never happened again in modern wars.

In modern wars, the winners do not really have any use for the inhabitants of the occupied territory, so even when they avoid to kill them, they will just steal in one form or another most of what they own and they will discriminate them in various ways.

Especially the Russians have a long history of stealing everything they could from their neighbors after winning any war against them and making any conquered people 2nd class citizens, who had to give up their language, culture and history, and replace that with praising their Russian conquerors, rebranded as "liberators".

So when faced with something like a Russian invasion, which is a real risk for any neighbor of Russia, there is only one way of survival, which is "defending the country".

This is not some theory devoid of content, like the propaganda that American soldiers should invade for instance Cuba, because this "defends the interests of their country" (which is code for defending the interests of a few ultra-rich people).

When you are in Europe, there is a non-null risk that you might be forced in the future to "defend your country", as the only means for your own survival.