I often think of what happened to Texas.

Amren:

When Mexico became independent of Spain in 1821 it tried to populate Texas by encouraging American immigration, but as Mr. Merry writes, “the newcomers rejected loyalty to Mexico and cast their devotion to their ethnic brethren in the United States.” Mexico woke up to what was happening and outlawed immigration in 1830, but could not stop it. Texas won independence from Mexico in 1836 and was recognized by the United States and the European powers.

An entire civilization actively working to bring about its own demise is I think unprecedented in history.