That's a super broad question that cannot fully be answered in a reddit post. You can argue that it was the post-WWII support of its allies that has lead America to be as wealthy and powerful as it is today. Or that it started with the ending of isolationist policies in the early 20th century. Or that it was Manifest Destiny that began it all. Or maybe even the Revolution and the establishment of independence. Heck, or it all began with the Seven Years' War.

In short, all of American history has, more or less, led it to become the wealthy and powerful country it is today. There are different theories as to what has been the most important factor in that process (land, resources, natural borders, people who settled here...) and many books try to expand on that.

I would suggest looking through the FAQ section in the sidebar and finding a way to narrow down your question if you want a detailed response.