Hmm. I don't know if I'd lay all the blame solely at Rhaegar and Lyanna's feet. I mean, they definitely created a tense situation. True, they could've defused it. However I'd say Brandon Stark riding down to King's Landing made an already bad situation worse. Then, the Mad King fanned the flames into full fledged war. Had he not murdered Brandon and his father Rickard, the war never would've happened.