I was outside walking my dogs on a mountain top when i noticed a "flashbulb" type light go off to the north west.The flashes started getting brighter and closer as they moved towards me.Altitude was about 50,000'. as the flashes got closer they became very bright, as bright as lightening!As the object got almost overhead i saw that the flashes were coming from an invisible (except for the flashes hitting it) cylinder shaped craft that was being swarmed by 20-30 smaller craft.I saw the "flashing" hit the opposite side of the object from me and saw that the cylinder was inside some kind of force field and that all the flashes were coming from some kind of electrical bursts hitting the outside of the force field.I had a monocular and used it to confirm what my eyes were seeing.The smaller craft were self illuminating and flashing off and on every second or so.The big(1,0000 yards long and 100 yards tall)craft was invisible to the naked eye except when the flashes hit the exterior or the force field.The crafts continued due east and then the crafts all did a slow 360 degree retrograde movement coming back towards me and then completing the circle and changing in heading to head south out over the bay of maine.As the crafts were changing directions i clearly saw 3 more smaller craft flying incredibly fast(mach 4 or 5?) and following the rest until they too caught up and joined the attacking swarm.The air speed of the whole group was very slow,maybe 100-200 mph and i watched the whole event in shock for almost 10 minutes.There was no noise.The flashes hitting the larger craft continued the whole time,some were too intense for the naked eye and cast a shadow on the ground where i was standing.About 30 minutes later a "us military aircraft" showed up and very slowly flew north towards where the crafts were first seen.This same us military aircraft is always around when strange aircraft or ufo's are spotted. my only logical deduction is that the smaller craft were attacking the larger one who appeared unhurt by the flashes hitting the exterior of the "force field"!One of the most incredible sighting i have ever had!