How many countries do you suppose we are obligated, by treaty, to come to the defense of if either party is attacked? According to the State Department, the answer is 55. Does the United States really have a strategic interest in the safety of all these countries, to the point that we would provide for their defense? Incidently, the list includes Cuba, which I found interesting.
http://m.state.gov/mc18610.htm
http://m.state.gov/mc18610.htm