What are the implications of swearing an oath to an individual leader, rather than to a nation?

What event brought the us into wwii? what conflicts happened after the us entered the war? i know the first part of this question, i need with the second part.

How did the completion of the transcontinental railroad change the lives of american citizens? a-it increased the wealth of all americans and enabled them to travel more.b-it made cross-country travel too expensive for ordinary americans.c-it decreased the safety of americans who moved to settle the plains.d-it shortened travel time between the east and west for all americans.

What would be considered a secondary source about the assasination of abraham lincoln