Nobunaga's victory at Okehazama was due to the surprise attack tactic. Historian Fujimoto Masayuki does not think so. In fact, In Paul Varley's article, "Oda Nobunaga, Guns, and Early Modern Warfare in Japan" (p. 114), Fujimoto suggests that there is no evidence to prove that Nobunaga victory was due to the surprise attack.
Fujimoto's book on Okehazama, Nobunaga no Tatakai: "Okehazama Nobunaga no Kishu Shinwa wa uso datta (p. 188-205) stated that the surprise attack is a total myth and it was started by Oze Hoan, the author of the Shinchoki. Then why was Hoan's biography on Nobunaga so popular and the surprise commonly excepted? It was easy to read, exciting, and a best seller during its day. I must warn you that Fujimoto wastes no time trashing Oze Hoan and it seems that it was personal.
Ota Mitsuaki's Okehazama no Shinjutsu (pp. 40-41) mentioned that the common theory's make-up and birth was started by Oze Hoan and the Nihon Senshi (published in 1910?).
From my own personal military experience and visiting both Okehazama battlefields, Fujimoto is dead wrong in my opinion. Nobunaga's victory was due to the surprise attack tactic. The surprise attack was the only choice Nobunaga had and he used it brilliantly. He had only one chance to succeed because a prolong guerrilla war with the Imagawa was not an option.
My personal military experience made my decision much easier to conclude that Nobunaga used the surprise attack against Imagawa Yoshimoto. The heavy rain that day forced the Imagawa troops to huddle and made it harder to scout the Oda. Nobunaga did not have the numbers. He was outnumbered almost twelve to one and Nobunaga had to use his resources wisely. Use the intelligence he received from Yanada Masatsuna and attack the enemy as soon as possible.
Nobunaga never used the surprise attack tactic again during the rest of his glorious career. Even Admiral Yamamoto prior to Pearl Harbor attack 1941, knew Nobunaga's surprise attack tactic was key to their success as written from his diaries.
I disagree with Fujimoto quite a bit and that is due to my military background (I have nothing against Fujimoto personally. He could be a great guy). I would like to give Fujimoto some sound advice. "All warfare is based on deception." If he was placed in Nobunaga's shoes during the battle, I will bet that even Fujimoto himself would use some sort of a surprise attack tactic against the Imagawa.
He failed to mention oral history which is still important in my opinion. It has its drawbacks, but oral history is still useful when used properly. Fujimoto is a great historian and the reason I enjoy his work because he is different from the others. I respect his opinion and he may be right in his own mind which does not bother me at all.
Tenka no tame!