Why is the U.S. such a powerful country?

Geography

1. Size: It is the 3rd largest in area and population. The two countries bigger than the US - Russia and Canada have way too much unusable land. The 2 countries bigger than US in population - India and China are still climbing up from the colossal destruction faced in the 19th century - early 20th century and also face severe resource constraints. Russia has still not settled in terms of governance. Brazil & Canada have too few people. That leaves US in a nice sweet spot.
2. Isolation: Guarded by 2 oceans and a frigid nation to the north, US never had to worry about a major war on its borders. Apart from the Pearl Harbor attack, most of the US military operations in the past century were far outside its territories. Most other nations bed with their fiercest competitors (China-Russia, India-China, India-Pakistan, China-Japan) leading to insecurity and poor decisions. 
3. Self-sufficient: US is probably the one country that is fairly self-sufficient in all fundamental resources. Unlike China, Japan, Germany or India, it has large quantities of oil & gas. Unlike the Middle East, it has plenty of water & agricultural land. Thus, throughout the 19th century, US grew unimpeded depending on Europe only for the luxury things.

People

1. Flight of the smart: Throughout the ages, US' remote location helped cherrypick the immigrants. Other than the Mexicans, everyone else had to travel long oceans to reach the US. That made the inhabitants self-selected -- the less ambitious ones were left behind. The puritans, then the Italians, Irish, then the East European Jews and then the Asians all added faster and faster growth as hard work was a matter of survival for each group (returning back was not an option). US got on to nuclear weapons and other key ideas on the back of its immigrants.
2. Education: For each wave of immigrants, education was a top priority. Whether it is the Puritan clergymen who created the Harvard or the Ashkenazi Jews who immigrated in 30s & 40s or the Asians who immigrated recently, getting to the top of ladder through education was both seemed possible and mandatory. Thus, US built itself as the world's education superpower with a huge concentration of top universities.
   
   
3. Naturally entrepreneurial: If you had to move this far and then stead a random piece of land in the middle, you need to be naturally entrepreneurial. Every country has its noble class. Since, Americans don't have Europe's aristocracy or Indian caste system, it created its own hierarchy - based on entrepreneurship. Getting rich became the mantra and people were ranked by their riches. It celebrated its entrepreneurs like no others.

Leadership

1. Right leaders at the right time: US leaders were quite good by world standards. Despite all their complaining, they never had a leader like Stalin, Saddam or Mao. More importantly, US was fortuitous in their timing. They didn't have someone like George W. Bush in 1861. FDR during Great Depression, Lincoln during Civil War, Eisenhower during post-war reconstruction, Theodore Roosevelt during the early 20th century expansion.
2. Stability: The present governments of China and India are less than 70 years old. US is about 240 years old and that kind of stability was lacking in much of the world.
3. The Wars: By the start of the WW-I, US had built up a massive economy. During the war, it finally announced the "secret" to the whole world. Although US faced some destruction, in relative terms it got far ahead of the rest of the world (most of which was obliterated). It was a victor in both the world wars and sparred of the ignominy faced by Germany and Japan