Lots of great answers here by lots of far more intelligent people than me. I will simply add that I've always been in awe of the power of phasers, especially how they were used in TOS. Such sophisticated energy weapons would be incredibly devastating, especially if mounted to satellites. A country could vaporize an enemy country or continent in minutes or seconds. I don't think the stun setting would be used much in today's warfare. There are non-lethal applications, too, but I don't think the people who would use the tech in such ways would actually be allowed access to it.