When the student is ready the master appears

As per the Indian mythology Arjuna is one of the best students. He is his guru Drona's favorite student. Krishna is the ultimate source of knowledge. He was Lord Vishnu himself, and available to Arjuna 24*7 as a friend, well wisher and god. Geeta is the master piece a conversation between Arjuna and Krishna. Tons of articles are available to understand Geeta. I am neither an expert in Geeta nor trying to explain Geeta.

Why did Krishna choose to explain it to Arjuna? Why did he choose to explain just before the mahabharat war? Was Arjuna fool or coward, who wanted to avoid the war?

Arjuna was definitely neither coward nor fool. He was one of the smartest person and best student of his teacher. He was brave and energetic and had proved it enough time. Krishna choose to explain Arjuna and not anybody else, because none other bothered about it. Probable none else had the intelligence and foresight to ask such questions. As questions are from Arjuna, Krishna choose to explain Arjuna instead of everybody. Arjuna did not have the ability to see his Viswaroop, he was able to see it after gifted with Divya chakshyu. So others were not clear about their task, but were not knowledgeable enough to ask such questions or understand the answer. So Krishna explained it to Arjuna.

Krishna explained it during war because Arjuna had the confusion during war. He did not ask it earlier, so Krishna did not explain it earlier.

So krishna the best friend, brother in law, well wisher and supreme lord of Arjuna is ready to answer question only when Arjuna has questions. So its not the presence of the master or getting a master but readiness of the student and having appropriate question can create a master piece.