The presence of the British troops in Boston roused all Warren's indignation. Overhearing one day some British officers saying that the Americans would not fight, he said to a friend:
"These fellows say we will not fight. By heavens, I hope I shall die up to my knees in their blood!"
Soon after, as he was passing the public gallows on the Neck, he overheard one of a group of officers say in an insulting tone:
"Go on, Warren; you will soon come to the gallows."
The young doctor turned, walked up to the officers, and said to them quietly:
"Which of you uttered those words."
They passed on without giving any reply. He had not long to wait for a proof that his countrymen would fight. April nineteenth, 1775, word was brought to him by a special messenger of the events which had occurred on the village green at Lexington. He called to his assistant, told him to take care of his patients, mounted his horse, and rode toward the scene of action.
"Keep up a brave heart!" he cried to a friend in passing. "They have begun it. That either party can do. And we will end it. That only one can do."
Riding fast, he was soon in the thick of the melée, and kept so close to the point of contact that a British musket ball struck a pin out of his hair close to one of his ears. Wherever the danger was greatest there was Warren, now a soldier joining in the fight, now a surgeon binding up wounds, now a citizen cheering on his fellows. From this day he made up his mind to perform his part in the coming contest as a soldier, not as a physician, nor in any civil capacity; and accordingly on the fourteenth of June, 1775, the Massachusetts legislature elected him "second Major General of the Massachusetts army." Before he had received his commission occurred the battle of Bunker Hill, June seventeenth. He passed the night previous in public service, for he was President of the Provincial Congress, but, on the seventeenth, when the congress met at Watertown, the president did not appear. Members knew where he was, for he had told his friends that he meant to take part in the impending movement.
It was a burning hot summer's day. After his night of labor, Warren threw himself on his bed, sick from a nervous headache. The booming of the guns summoned him forth, and shortly before the first assault he was on the field ready to serve.