Final answer:
The main idea is that people have the right to establish democratic governments of their own choice, a principle outlined in the Atlantic Charter and asserted in the Declaration of Liberated Europe to rebuild nations after the fall of Nazism and fascism.
Step-by-step explanation:
The main idea of the underlined text in the passage from the Declaration of Liberated Europe is that all people have a right to create democratic institutions of their own choice. This principle, affirmed in the Atlantic Charter, empowers the liberated peoples to dismantle the remnants of Nazism and fascism and establish self-governance and sovereign rights reclaimed from aggressor nations.
Following World War I, disillusionment with liberal democracies led to the rise of fascism, a movement prioritizing nationalism and authoritarian rule, as seen in Germany, Italy, and Spain. However, the counter movement advocated for restoring democratic governments and protecting individual freedoms, such as those laid out in the Declaration of Liberated Europe and echoed by the democratic principles articulated in the preamble of the U.S. Declaration of Independence.