What was the MOST important or historically significant change or development that occurred in the United States during the late 19th and early 20th centuries? Why?

The late 19th and early 20th century was a period of transition architecturally, marking the entrance into a new era of building. This was the beginning of forward looking architectural design with styles not based on previous building forms.

montesquieu took the view that the roman republic had powers separated so that no one could usurp complete power. in the british constitutional system, montesquieu discerned a separation of powers among the monarch, parliament, and the courts of law.