A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.
Can you explain?
The Second Amendment was written with the expectation that the United States would not have a standing army. That’s why why the first clause is there. The well-regulated Militia in this context means (potentially) every citizen with a gun willing to fight in the defense of the state. Essentially, the amendment guarantees every capable and trustworthy person access to weapons of war for the explicit reason that if a large portion of the populace is armed, then it becomes impossible to forcefully occupy the country.