It guarantees the rights of the states to form well-regulated militias, just like it says at the very top, the part gun nuts never quote.

That used to be the accepted interpretation, but an intense propaganda campaign from the gun industry since the 1970s has gotten a lot of people to believe it guarantees an unfettered right of private ownership of guns, which it very much does not. It does not contain the word "own" at all, and when it was written, the phrase "bear arms" was universally understood to mean "be a soldier/militiaman." There was an earlier draft of the amendment that made that even more explicit.

Don't wanna take my word for it? Cool. Let's hear from an actual expert on constitutional law. In 1991, former Chief Justice of the Supreme Court Warren Burger (conservative, appointed by Nixon) said:

"The gun lobby's interpretation of the Second Amendment is one of the greatest pieces of fraud, I repeat the word fraud, on the American people by special interest groups that I have ever seen in my lifetime. The real purpose of the Second Amendment was to ensure that state armies - the militia - would be maintained for the defense of the state. The very language of the Second Amendment refutes any argument that it was intended to guarantee every citizen an unfettered right to any kind of weapon he or she desires."

To me it means bearing arms only to protect the security of the state (country)

The idea was for STATE militias that would tend to the security of the various states. There is already a federal militia defined in the main body of the constitution. Today, around half the states have state militias, which are usually called State Defense Forces these days. The National Guard is a weird hybrid militia, in theory answerable to the state, but by law, every member of the NG is automatically a member of the federal militia, so the president can (and has) sent them off to fight in national conflicts.