On Jan 16 1790, he bought (as was customary at the time) the rank of lieutenant and formed his own company. He was later promoted to captain and transferred to the 49th (Hertfordshire) Regiment of Foot.
He became acting commanding officer of the regiment, succeeding the retiring Lieutenant-Colenel Fredrick Keppel, who was supposedly threatened with a court martial and probably dismissal.
He and his regiment were originally assigned to Montreal. Almost immediately, he was faced with a challenge: seven of his men deserted and fled into America. He handled it by sending a party to capture the men, despite having no jurisdiction on American soil.
Despite his limited education, Brock managed to implement an effective defensive strategy and infrastructure mostly from reading from his ever-growing library. He ran into some conflicts with civilian authorities (Thomas Dunn) and Governor General (Sir George Prevost) but managed to sort them out.
Between 1805 to 1812 tensions between the British and Americans increased dramatically, until 1812 when war broke out.
Brock collaborated with the Native allies led by Tecumseh and used several 'bluffs' to make the Americans at the fort to think that their army was much larger than it really was. Although the Americans realy outnumbered them almost 2 to 1, he managed to get them to surrender with almost no real fighting (he lost no men, and only 2 were wounded) This victory was very significant to both sides: it drastically raised the morale of the Canadians, and it prompted American natives to attack other American forts.
In what was known as "The most lopsided battle in history", Brock's was forced to lead an inexperienced militia general and a small regular army against a large American army with land advantages and full cover. Somehow, the battle was a victory, although Brock himself was killed.
He was posthumously named as "The Hero of Upper Canada" and voted 28th on The Greatest Canadian, even though he isn't actually Canadian.