The military history of Canada spans centuries of conflicts within the country, as well as international engagements involving the Canadian military. The Indigenous nations of Canada engaged in conflicts with one another for millennia. The arrival of European settlers in the 17th century led to new alliances and hostilities among Indigenous nations and colonial powers, leading to conflicts such as the Beaver Wars.

The late 17th and 18th centuries saw four major British-French conflicts fought in Canada, culminating with the British conquest of New France in 1760. This reshaped the region and contributed to the American Revolutionary War, during which American attempts to seize Quebec and spark a revolt in Nova Scotia failed.

The 19th century brought both external threats and internal challenges to British North America. While its colonies repelled American invasions during the War of 1812, the Rebellions of 1837–1838 and Fenian raids highlighted the need for militia reforms and contributed to Canadian Confederation in 1867. The end of the century saw Canadian units become involved in British imperial conflicts like the Nile Expedition and Second Boer War.

In the 20th century, Canada played a significant role in both World Wars. After World War II, it embraced multilateralism, with its military participating in international coalitions and peacekeeping missions, including the Korean War and the Gulf War. In the 21st century, Canada has continued its involvement in multilateral military coalitions, contributing to missions in the Greater Middle East and Mali.

Warfare existed in all regions and waxed in intensity, frequency and decisiveness. It was even common in subarctic areas that had sufficient population density. However, Inuit groups in the extreme northern Arctic typically avoided direct warfare due to their small populations, relying on traditional law to resolve conflicts. Conflict was waged for economic and political reasons, such as asserting their tribal independence, securing resources and territory, exacting tribute, and controlling trade routes. Additionally, conflicts arose for personal and tribal honour, seeking revenge for perceived wrongs.

In pre-contact Canada, Indigenous warriors relied primarily on the bow and arrow, having honed their archery skills through their hunting practices. Knives, hatchets/tomahawks and warclubs were used for hand-to-hand combat. Some conflicts took place over great distances, with a few military expeditions travelling as far as 1,200 to 1,600 kilometres (750 to 990 mi).

Warfare tended to be formal and ritualistic, resulting in few casualties. However, some violent conflicts occurred, including the complete genocide of some First Nations groups by others, such as the displacement of the Dorset of Newfoundland by the Beothuk. The St. Lawrence Valley Iroquois were also almost completely displaced, likely due to warfare with their neighbours the Algonquin. The threat of conflict impacted how some groups lived, with Algonquian and Iroquois groups residing in fortified villages with layers of defences and wooden palisades at least 10-metre-tall (33 ft) by 1000 CE.

Captives from battles were not always killed. Tribes frequently adopted them to replenish lost warriors or used them for prisoner exchanges. Slavery was common among the Pacific Northwest Coast's Indigenous people like the Tlingit and Haida, with around a quarter of the region's population being enslaved. In certain societies, slavery was hereditary, with slaves and their descendants being prisoners of war.