Short Answer 
The post-Civil War era, known as the Reconstruction Era, marked a period of national rebuilding focusing on economic recovery and social reforms, particularly benefiting African American laborers. This transitioned into the Gilded Age, characterized by rapid industrial growth and technological advancements, leading to significant economic boosts for the country, followed by territorial expansion that established the U.S. as a global power following events like the Spanish-American War.
Step 1: Understanding the Post-Civil War Era
After the end of the Civil War in 1865, the United States entered a significant phase known as the Reconstruction Era. This period marked the rebuilding of the nation, particularly focusing on economic recovery and social reforms. Major changes took place leading to the influx of African American laborers, which vastly increased the agricultural output and paved the way for greater prosperity in the North and West.
Step 2: Embracing Economic Growth and Industrialization
As the United States transitioned into the Gilded Age, the country experienced rapid industrial growth and technological advancements. This era saw a surge in new technologies that fueled industrial production. Key contributors to this growth included:
- Innovative developments in steel production
- The expansion of telegraph communications
- Improvements in transportation, particularly with railroads
- Abundant natural resources that supported industrial needs
These advancements contributed to what is known as the Second Industrial Revolution, greatly boosting the nation’s economy.
Step 3: Expanding Influence and Territorial Gains
With political stability achieved, the U.S. began to focus on international affairs and territorial expansion. The country established a minor empire through key acquisitions, including:
- Alaska in 1867
- Puerto Rico and various smaller islands
- The annexation of Hawaii in 1898
The culmination of this expansion was seen post the Spanish-American War in 1898, which resulted in the U.S. gaining control over territories like Cuba and the Philippines, thereby establishing America as a prominent global power.