US intervention

I wonder why the US spend so much energy, time and money in countries like Israel, Afghanistan, Korea, Japan and Iraq. What does it gain from intervening in those countries' politics? Please, no hateful messages. I'm curious about it. I would appreciate American and international replies.

If the interest is to ensure national security, isn't better to spend the money on a better defense system in and around the US? I agree with the decision to invade and paralyze countries that intend to use nuclear or weapons of mass destruction on others, like what some countries thought Iraq had. Now that it's proven otherwise, shouldn't the US get out now and use that money to improve its own economy? I think that's what the US is doing but I guess my question is why only now?

Actually, I'm nore interested in knowing why the US stations troops in countries like South Korea, Japan, I believe Israel too, and some countries in the world. What's the point?

Sorry if my questions sound stupid, as my knowledge of history is almost next to none. I'm reading and trying to understand better and this is part of the effort.

After cold war USA made himself  a most powerful state in world . USA  goverment trying to keep this situation and destroy ( or make all possible problems ) all state,  who may be threat for USA. As result world lost any respect to international law and many land trying to make nuclar bomb as insuranse. It may be dangerous for all world 

Sorry, Isu, I didn't see your post.  Your point is well taken.   - A.

Caribean crisis began ,  when  USA put her missles in Tyrekey. Khrushow was angry and decide to put soviet missiles in Cuba.
Sorry, but I scared to hear about  troops from another side of earth , massed near border of my land.