The Bhagavad Gita is indeed a part and parcel of the Mahabharata. In order to remove Arjunas bewilderment due to sudden preponderance of Tamas in the battlefield of Kurukshetra, LordKrishna sang to him the message of the Upanishad. The Gita contains His message that as a Kshatriya Arjuna should perform his duty of fighting against evil, the Kauravas for the sake of duty itself.