Final answer:
The city known as the "city of the big shoulders" is Chicago, a nickname originating from a poem that highlights its industrial prowess. Chicago's population grew dramatically from about 100,000 to 2 million between the Civil War and World War I.
Step-by-step explanation:
The "city of the big shoulders" that rocketed to prominence in barely 50 years is Chicago. This nickname for Chicago comes from a poem by Carl Sandburg, which celebrated the city's industrial strength and its critical role as a hub for agricultural and industrial work, thanks to its unique geographic location. The rapid growth of Chicago is well-documented, especially during the period between the Civil War and World War I, when its population soared from approximately 100,000 to 2 million residents, earning it the moniker of the "Second City."