Nuclear proliferation is not a question of the sovereignty of foreign lands, it is a question of national security for the United States. We must not allow other nations to gain the sort of leverage over us that nuclear weapons provide, especially not minor powers such as Iran or North Korea.
That said, I think that, ultimately, stopping nuclear proliferation is a noble but untenable goal; nations will do what is in their best interest, and it is very, very much in the best interest of Iran to build nukes, and likewise it is in the interest of certain great powers that Iran have nukes. These are forces that cannot be restrained; it is like trying to stop the advacement of technology.
Certainly, attempts to stop the spread of nukes should be made, but they are no substitute to developing a first-strike capability against all other nuclear powers and developing a missile shield.