At what point then is the approach of danger to be expected? I answer, if it ever reach us, it must spring up amongst us. It cannot come from abroad. If destruction be our lot, we must ourselves be its author and finisher. As a nation of freemen, we must live through all time, or die by suicide.

Abraham Lincoln (1809-1865) American lawyer, politician, US President (1861-65)

Speech, Young Men’s Lyceum, Springfield, Illinois (27 Jan 1838)
    (Source)

This seems to be the source of this far more prosaic, and spurious, Lincoln quote: "America will never be destroyed from the outside. If we falter and lose our freedoms, it will be because we destroyed ourselves."