America will never be destroyed from the outside. If we falter and lose our freedoms, it will be because we destroyed ourselves.

This quote means that it is impossible for forces outside of America to completely destroy the country; it will only happen if people within the country turn on their own freedoms and values. It implies that the power to protect America lies in the hands of its citizens, and if they are not vigilant and careful to preserve their freedom, then America will no longer exist.