Article II

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.

This was written at at time when the United States had no standing army.

Two things suggest themselves. First, the right to bear arms is a function of a militia, created to defend the state. In other words, citizens carry arms as members of a citizen's army. Not just any old army, but a "well regulated" one. If someone is going to bear arms, then, it should be in the context of membership in a military unit under the ultimate control of the civilian government.

The so-called "right to bear arms" is not about the security of an individual; it is about the "security of a free State". An individual, bearing arms, outside the control of the state is quite easily construed as a threat to the State -- certainly bank robbers and drug dealers are seen as such.

Second: While the Constitution mentions an army and navy, it does not specifically establish them. There is a suggestion in this article that the militia is voluntary (almost by definition) but that the army and navy should also be a citizen's army and navy, as opposed to a professional one.

In any case, if "national defense" and "national security" are truly issues, then it would be more effective to require that everyone spend some time in the military. This would mean that everyone would be trained to defend their country, rather than having to depend on a few professionals. Paradoxically, citizens' army might also limit the power of the military since citizens might be less inclined to put themselves or their children in harm's way at the whim to satisfy the ambition of a political leader than would professional soliders.