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 (1838-01-27), Young Men’s Lyceum, Springfield, Illinois
    (Source)

Lincoln goes on to describe growing issues of lawlessness and mob justice.

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.