Story behind:
After the fall of Bheeshma, Drona was leading kauravas. Drona spread fear and destruction in the Pandava army by his relentless attacks. "O Arjuna," said Krishna, "there is none that can defeat this Drona, fighting according to the strict rules of war. We cannot cope with him unless dharma is discarded. We have no other way open. There is but one thing that will make him desist from fighting. If he hears that Aswatthama is dead, Drona will lose all interest in life and throw down his weapons. Someone must therefore tell Drona that Aswatthama has been slain." Arjuna shrank in horror at the proposal, as he could not bring himself to tell a lie. Those who were nearby with him also rejected the idea, for no one was minded to be a party to deceit. Yudhishthira stood for a while reflecting deeply. "I shall bear the burden of this sin," he said. Bhima then kills an elephant named Aswatthama and goes to Drona and declares "I have killed Aswatthama" feeling ashamed. Drona heard these words as he was in the act of discharging a Brahmastra. "Yudhishthira, is it true my son has been slain?" Dronacharya asked addressing Dharmaputra. The acharya thought that Yudhishthira would not utter an untruth whatever reason it might be. When Drona asked thus, Krishna was terribly perturbed. "If Yudhishthira fails us now and shrinks from uttering an untruth, we are lost. Drona's Brahmastra is of unquenchable potency and the Pandavas will be destroyed," he said. And Yudhishthira himself stood trembling in horror of what he was about to do, but within him also was the desire to win. "Let it be my sin," he said to himself and hardened his heart, and said aloud: "Yes, it is true that Aswatthama has been killed." But, as he was saying it, he felt again the disgrace of it and added in a low and tremulous voice, "Aswatthama, the elephant" words which were however drowned in the din and were not heard by Drona. He says "Aswatthama hatah - kunjarah" kunjarah in very low voice which didnt reach drona. Listening this Drona threw his weapons away and sat down in yoga on the floor of his chariot. At the same moment Dhrishtadyumna beheads Dronacharya.
In my opinion hiding the truth knowingly to decieve some one is equal to lie. So I think he did lie though the words uttered are not. When the words of untruth came out of Yudhishthira's mouth, the wheels of his chariot, which until then always stood and moved four inches above the ground and never touched it at once came down and touched the earth. Yudhishthira, who till then had stood apart from the world so full of untruth, suddenly became of the earth, earthy. He was compared to lord Shiva who swallowed poison to save the world and Lord Rama who had to kill Vali to save his friend disregarding the rules of fairplay.
