For many years the small settlements on the north shore of the St. Lawrence River had been the subject of frequent raids by the Mohawk, one of the five nations that comprised the Iroquois Confederacy.

Courcelle arrived in New France in September 1665 and "breathing nothing but war" immediately involved himself with supervising the construction of forts along the Richelieu River, the main route used by the Mohawk in their raids on French settlements.

[4] Courcelle's main contributions to the colony during his tenure were the actions he took to maintain peace, resolve conflict between the Iroquois and the Algonquin, sustain the fur trade, and raise "the prestige of the French in the eyes" of the Indigenous population.

When the fur trade was threatened by renewed hostility between the Iroquois and the Algonquin, Courcelle mounted a show of force by ascending the St. Lawrence River to Lake Ontario with 56 volunteers and a large bateau, demonstrating that the French could attack the western Iroquois as easily as they had attacked the Mohawk.

In 1669, following the orders of Louis XIV, Courcelle established militia companies in which all able-bodied men between the ages of 16 and 60 were drilled in the use of arms.

Courcelle also encouraged the exploration of the Great Lakes watershed and beyond by La Salle, Nicholas Perrot, Louis Jolliet, Jacques Marquette, and Daumont de Saint-Lusson.