It is not about physically conquering places anymore. The world wars mostly phased that out. What it is about is placing our ideals, morals, and culture above others and making them adhere to it. Thru economic, cultural pressure, and military might.
It is what makes trump so dangerous. He endangers the world standing and believe in the US on a global scale. A scale that was forged with us at the center.
If countries stop looking to the US for answers and/or solutions it weakens our national buying power and influence. Which would be basically what has happened to every world power from Rome up to Britain