He chose his profession early in life. For him there could be only two careers, the navy or the army. Guernsey men, from time immemorial, had favored the services as a means of earning their living, for the love of adventure was ingrained in the people. Besides, Brock had two brothers in the army. One brother, Ferdinand, had been in the 60th Regiment, and when Isaac was a lad of ten, had given his life at the defence of Baton Rouge, on the Mississippi, fighting against the colonial revolutionists. The other, John Brock, was a captain in the 8th, known as the King’s Regiment, and probably with the idea of being near his brother, Isaac in 1785 purchased a commission as ensign in the 8th. Thus he had in John a hand and mind steadied and practised by reason of ten years’ service to guide and help him in the career he had chosen. Isaac was keenly enthusiastic about this new life, and his brother’s example spurred in him the ambition to be a distinguished soldier. His love for history and his liking for serious reading stood him in good stead. He had had, perhaps, too much sport and too little study in those Guernsey days. He allotted his time differently now, and sedulously spent some hours each day locked in with his books. He was wise enough to know that he was not too well-equipped for his work. These were the years when his mind was receptive and plastic, and he used them well. He served five years and purchased his lieutenancy in 1790, when he was twenty-one. These were uneventful and quiet days, but they were days of preparation. Barrack-room and camp taught him the essential elements of soldierliness. He returned to Guernsey, for he had been quartered in England, and raised an independent company. This he commanded with the rank of captain, being placed on half-pay. The quietness and sameness of soldiering in England palled on him, however, and in the next year he arranged a transfer to the 49th Regiment, then quartered in the Barbadoes. These were the men whom he was to learn to love, and many of whom fought with him when, some years later, he received his death wound.