This "Yes, I shall fight", this "I", and the former "I" - "I shall not fight" - so there is vast difference. The former "I" is the representative of mental speculation, when Arjuna decided that "I shall not fight. They are my relatives, they are my brothers; I cannot fight with them for the matter of kingdom. Rather, I shall forego; I shall become a beggar. I shall... I don't want this kingdom." He argued like that. But after reading Bhagavad-gita, he said that "My illusion is now removed." Nasto mohah smrtir labdha: "My illusion is now removed, and I have got my consciousness by Your mercy. By Your mercy."
Lecture on Bhagavad-gita 2.55-58, New York, April 15, 1966