BG 1.19: The terrific sound thundered across the sky and the earth, and shattered the hearts of your sons, O Dhritarasthra.
BG 1.20: At that time, the son of Pandu, Arjun, who had the insignia of Hanuman on the flag of his chariot, took up his bow. Seeing your sons arrayed against him, O King, Arjun then spoke the following words to Shree Krishna.
Commentary
Sanjay conveyed to Dhritarashtra, that the tremendous sound of the various conch shells from the Pandava army was shattering the hearts of his sons. Whereas, he did not mention any such reaction from the Pandavas, when the Kauravas were creating a commotion. The Kauravas were fearful, as their conscience pricked them for their crimes and misdeeds. They were relying solely on their physical strength to fight the war. However, the Pandavas were confident and felt protected, as the Supreme Lord Shree Krishna was by their side, their victory was definite.
Here Sanjay addresses Arjun by another name, "Kapi Dhwaj," which means "Monkey Bannered." This denotes the presence of the mighty Monkey God; Hanuman on Arjun's chariot. It so happened that, once Arjun became very boastful of his archery skills and quipped at Shree Krishna. He said, "I do not understand why during Lord Rama's time, the monkeys worked so hard to make a bridge from India to Lanka with heavy stones? If I was there, I would have made a bridge of arrows." The Omniscient Lord asked him, "Alright, go ahead show me your bridge."
Very skillfully Arjun showered thousands of arrows and made a huge bridge. Now, it was time to test it. Shree Krishna called upon great Hanuman for the job. As soon as Hanuman started walking on the bridge, it started crumbling under his feet. Arjun realized his folly; his bridge of arrows could not have upheld the weight of Lord Rama's huge army. He asked for their forgiveness. Subsequently, Hanuman gave Arjun lessons on being humble and never be proud of his skills. He also granted Arjun a boon that, during the great war, he would seat himself on Arjun's chariot. Therefore, Arjun's chariot flag carried the insignia of the great Hanuman.