We have troops positioned all across the globe for both reasons. We have the position of being turned to in times of need by most nations because of our power, wealth and generosity. Almost always, before we have to enter a nation in force, we have had scores of people there to provide education, teach farming techniques, administer medical assistance, provide tons of food, help in governmental problems, studying the local traditions, beliefs, government and laws.
As usual with any leading nation, we also have businesses there, looking for markets, export potentials, tax breaks, low wage employees and sniffing the over all political wind. Businesses don’t want a repeat of Saudi Arabia and the like were they spend billions establishing companies, factories and so on, only to get forced out by the local government who takes over the then fully functional operations.
We can go in for a dual purpose to defend a people and protect our financial interests.
We have a greater interest in oil, of course, since our financial institutions insist on basing loans and investments on the volatile price per barrel. (Which means, the higher the price, the more money they make, even though by doing so, they damage the National economy when pump prices go up.)
I’m not sure why we suddenly switched from the rich oil fields of Texas to importing crude so much. (Unless because the arrogance of the Texans over oil money just ticked off everyone in government.) Plus, we installed the Alaskan pipeline to cheaply draw on Alaskan oil.
Since OPEC provides us with most of our oil, it is more understandable that we go in to any of the nations involved when they are under attack to not only protect our interests, but protect our allies.
We do have many allies across the globe who we are sworn to assist.