The United States has a right under international law to protect the safety and security of its citizens--even to the extent of using armed force as a last resort.
If the lives or safety of American citizens in Grenada were in danger prior to our military action, the president acted properly. Since news coverage has been blacked out or selectively permitted, only the president can answer that question. If our principal motivation was political, i.e., to replace a government we didn't like or impose one we do like, then our actions went beyond acceptable bounds.
A president of the United States cannot arbitrarily decide to invade another country when he does not like the policies of that nation. If this were the case, the United States would be invading half the nations of the world on a regular basis.
It is difficult, if not impossible, to make an argument that activities in Grenada (all of whose citizens could occupy the Rose Bowl) jeopardized United States national security. If those activities jeopardized regional or hemispherical security, then the Organization of American States should have been consulted and involved. Their judgment as to whether their security is at stake is at least as valuable as ours.
The larger question is the militarization of our foreign policy--the dangerous trend in this administration. We have a limited number of Marines and we cannot invade and indefinitely occupy every trouble spot on the globe--particularly the tiny ones. Surely somebody in authority can begin to think soon about preventive steps of a more positive nature.