Well. I'll try to be brief, as requested. I believe (or 'know', in the Schotzian sense of the word) that the assassination was a Cold War coup d'état, instigated and perpetrated at the highest levels of the US Government's power groups. It's basically Vincent Salandria's "model of explanation", probably best expressed in his "False Mystery" address at the 1998 COPA convention. I heartily recommend reading the FM essay to any who might be interested, and of course it states that position far better than I could. I would post a link, but as this is my first day here at jfkessentials I'm not sure of the protocols re linking materials, so I'll just note that Salandria's FM address is easily 'googleable'; I think it can be found at John Kelin's website as well.
I suppose another way of putting it, in a 'whodunit' fashion, is that JFK was murdered (and the crime covered up) by the military/intelligence/industrial/media complex. Eisenhower got it partially right, but unfortunately didn't DO anything about it, and instead left it for Kennedy to grapple with. No favor there, to put it mildly.
That's about as succinct as I can put it. I sometimes say I'm of the "Salandria/Garrison/Douglass school" re 'conspiracy realism'. While I don't necessarily agree with 100% of the entirety of those authors' work, any differences are insignificant in a 'big picture' sense.
Thanks for allowing me the opportunity to opine.
All the best,
AE