The US is part of the United Nations
This could be an issue in light of the fact that the delegates wouldn't have the capacity to "control" the President if the president could fire judges from the Supreme Court on the off chance that he didn't care for a decision they made.
A Supreme Court Justice might be arraigned by the House of Representatives and expelled from office if indicted in a Senate trial, however just for similar sorts of offenses that would trigger prosecution procedures for some other government official under Articles I and II of the Constitution.
They would travel on foot
The answer should be:
Animal & human power were generally replaced with steam power.
