The global measure of a nation's wealth is GDP. In particular GDP per capita. This chart shows US GDP per capita from 1700 until 2008 versus several other countries: UK, Japan, Germany, China and India.
A couple of things to point out...
1. The United States, even when we were the 13 Colonies, was a wealthy area on par with most of Europe.
2. The United States was a massive exporter of agricultural goods and raw resources throughout its existence and after the Industrial Revolution it became a large exporter of manufactured goods as well.
3. The United States operated the world's largest mercantile fleet almost from the beginning of the nation's existence. Our interventions along the Barbary Coast as well as early conflicts with Britain and France all arose from issues pertaining to the freedom of American registered merchantmen.
4. WW2 certainly catapulted the US far ahead of the rest of the world, but the rise of the US to be the preeminent economic power began after the Civil War and by the turn of the 20th century, in terms of total GDP, the US surpassed the British Empire as the world's largest economy as illustrated in this graph:
The rest of your questions are a little more economics based and there are plenty of theories around them. However, the foundation of wealth building is taking a basic resource and "adding value". One can add value in many ways from turning raw resources into finished goods or by taking a resource abundant in one area and transporting it to where it is needed. Much of the early American economy was based on the latter where cotton and wheat from the United States was needed to fuel European industry.