How did the U.S expand from a country affected by a civil war to a "new empire" at the turn of the XXth century?

To introduce this blog, i'd like to start with a brief history about the Civil War. It was a fight in which the Northern States squared off against the Southern States. One of the reasons for this was that the economic interests were different in each side. The South was largely comprised of small and large plantations that grew crops such as cotton which were labor intensive. The north was more of a manufacturing center, using raw materials to create finished goods. Slavery had been abolished in the north but continued in the south, they used afroamerican people to do all the work without any reward.

By 1860 the conflict between the North and the south became stronger and when Abraham Lincoln was elected president, the Civil War exploded as he vowed to stop the spread of slavery to new territories.

The Civil War lasted from 1861 until 1865 and it was the bloodiest war ever in the U.S. There was about 530.000 deaths, and many more were wounded. Both the north and the south experienced major victories and defeats. However, in 1864 with the taking of Atlanta, the North had gained the upper hand and the war would officially end on April 9, 1865.

At the end of the war, uncertainty gripped both North and South. An entire social system and much of the South's wealth had been destroyed. The Confederacy was practically dead. 

Years after the war, America's industry had an incredible growth. The North began to control the congress, started passing Laws to protect northern products against cheap european goods and helped found American industrial complexes. Old industries expanded and a lot of new ones, including oil refining, steel manufacturing, and electrical power, emerged. Railroads expanded bringing even remote parts of the country into a national market economy. The U.S experiences an industrial revolution that changed the way millions of people worked and where they lived. They experienced the migrations of a lot of people from the rural America to the nation`s rapidly growing cities.

Taking into account what i just said and what i learnt in class, there were two interests that guide America in its pursuit of foreign interests, which led the country to become a new empire. The first one is to pursue favorable trade agreements and aligments to acquire territory and to have power over the Pacific and the Caribbean. And second they wanted to spread Christian and democratic ideals within the continent. These interests are shown when the U.S had an open door policy with China, support for the Panama Canal Project through doctrines such as the Monroe Doctrine and its influence in Latin America, economic expansionism investing in foreign territories, and its belief in the Manifest Destiny (the idea that America was destined to stretch from the Atlantic to the Pacific Oceans).

References: 

• http://www.tredegar.org/legacies-civil-war.aspx. Taken in March 8 2013. 
• KELLY, Martin. http://americanhistory.about.com/od/civilwarmenu/a/civiloverview.htm. Taken in March 8 2013. 
• http://www.loc.gov/teachers/classroommaterials/presentationsandactivities/presentations/timeline/progress/. Taken in March 9 2013. 
• Power Point presentation by LUIS MONROY.