First of all, it isn't exactly news that a nation looks out of its national interests; that's constitutionally what most countries, and their leadership, are bound to do.

Now then, as for our policy for the 20th century, for a large of the time, we have been quite isolationist. For more than half of it, we have been fighting this:



(and yes, add three zero's to those numbers, as the chart indicates) ...and here is the corresponding article:

http://www.hawaii.edu/powerkills/COM.ART.HTM

It's often forgotten, with people in the rush to attack the US, exactly what others have been doing during their tenure in the world...