Is there any relationship between Zen and Shingon besides them both being schools of Buddhism and both being Japanese forms of Buddhism? What I mean is was there any cross fertilization between these two traditions in Japan and did they influence each other in any way? Also what was the historical relationship between these schools--cooperation and mutual respect, rivalry or enmity, or merely indifference?
I ask because I once had someone tell me that Shingon was basically Japanese Esoteric/Tantric Buddhism with Zen influences....but I am not sure if that is accurate or not. Thanks in advance.