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Canada helped develop the world’s first nuclear reactors and nuclear arms. During the Second World War, Canada participated in British research to create an atomic weapon. In 1943, the British nuclear weapons program merged with its American equivalent, the Manhattan Project. Canada’s main contribution was the Montreal Laboratory, which later became the Chalk River Laboratory. (See Nuclear Research Establishments). This Allied war effort produced the atomic bombs dropped on the Japanese cities of Hiroshima and Nagasaki in 1945. It also led to the development of Canada’s nuclear energy industry.

Click here for definitions of key terms used in this article.

The Battle of Passchendaele, also known as the Third Battle of Ypres, was fought during the First World War from 31 July to 10 November 1917. The battle took place on the Ypres salient on the Western Front, in Belgium, where German and Allied armies had been deadlocked for three years. On 31 July, the British began a new offensive, attempting to break through German lines by capturing a ridge near the ruined village of Passchendaele. After British, Australian and New Zealand troops launched failed assaults, the Canadian Corps joined the battle on 26 October. The Canadians captured the ridge on 6 November, despite heavy rain and shelling that turned the battlefield into a quagmire. Nearly 16,000 Canadians were killed or wounded. The Battle of Passchendaele did nothing to help the Allied effort and became a symbol of the senseless slaughter of the First World War.

The Front de libération du Québec (FLQ) was a militant Quebec independence movement that used terrorism to try and achieve an independent and socialist Quebec. FLQ members — or felquistes — were responsible for more than 200 bombings and dozens of robberies between 1963 and 1970 that left six people dead. Their actions culminated in the kidnapping of British trade commissioner James Cross and the kidnapping and subsequent murder of Quebec cabinet minister Pierre Laporte, in what became known as the October Crisis.

The Battle of the Atlantic, from 1939 to 1945, was the longest continuous battle of the Second World War. Canada played a key role in the Allied struggle for control of the North Atlantic, as German submarines worked furiously to cripple the convoys shipping crucial supplies to Europe. Victory was costly: more than 70,000 Allied seamen, merchant mariners and airmen lost their lives, including approximately 4,400 from Canada and Newfoundland. Many civilians also lost their lives, including 136 passengers of the ferry SS Caribou.

For 45 years, the Canadian government investigated unidentified flying objects (UFOs). Several of its departments and agencies collected sighting reports of UFOs in Canadian airspace from 1950 to 1995. These investigations started during the Cold War, spurred by fears of Soviet incursions. What began as a military question eventually became a scientific one. From the start, however, the government was reluctant to study this topic. It devoted few resources to it, believing UFOs to be natural phenomena or the products of “delusional” minds. By contrast, many Canadian citizens were eager for information about UFOs. Citizens started their own investigations and petitioned the government for action. In 1995, due to budget cuts, the government stopped collecting reports altogether. For their part, citizen enthusiasts have continued to investigate UFOs.

The First Canadian Army was a field army formed in 1942 during the Second World War. It became an international force in 1943, when Allied units were added to keep the army up to strength. The First Canadian Army was commanded by A.G.L. McNaughton and H.D.G. “Harry” Crerar.

The Ministère de la Marine is the section of the French government that administered Canada during its last 100 years as a French colony. The Ministère de la Marine — variously described as a ministry, department, or secretariat of state — administered France’s navy, colonies and seaborne trade.

The Battle of the Scheldt was fought in northern Belgium and southwestern Netherlands in 1944 during the Second World War. It was part of the Allied campaign to liberate northwestern Europe and defeat Nazi Germany. The First Canadian Army played a crucial role in clearing the Scheldt of German forces, opening crucial supply lines via the port of Antwerp. However, this victory came at a cost. The Allies suffered nearly 13,000 casualties during the battle, including more than 6,300 Canadians.

Known as the “Queen of Battle,” the infantry is the branch of the army that provides its primary fighters. The main responsibility of infantry soldiers is to “close with and destroy the enemy.” Although they are trained, armed and equipped to fight on foot, infantry soldiers are usually transported to the battlefield by other means. Infantry soldiers can also specialize as light, mechanized, airmobile, airborne and other types. The characteristics of infantry are mobility, firepower, flexibility, communications and vulnerability (to enemy action). Infantry soldiers are trained in a wide range of individual and crew-served weapons and work with the all-arms team of reconnaissance, armour, artillery, air defence, engineers, tactical aviation and other combat specialists. Except for a brief time during the feudal period (when cavalry dominated), the infantry has been the largest single component of armies since ancient times. In Canada, the infantry has always been the army’s largest element.

Post-traumatic stress disorder (PTSD) is a mental illness that affects individuals exposed to trauma (although not all people exposed to trauma develop PTSD). Studies suggest that over 70 per cent of Canadians have been exposed to at least one traumatic event in their lifetime, and that nearly 1 out of 10 Canadians may develop PTSD at some point in their lives. PTSD can affect adults and children and can appear months or even years after exposure to the trauma.

Canada’s longest Second World War army campaign was in Italy. Canadian forces served in the heat, snow and mud of the grinding, nearly two-year Allied battle across Sicily and up the Italian peninsula—prying the country from Germany's grip, at a cost of more than 26,000 Canadian casualties.

The war in Afghanistan (2001–14) was Canada’s longest war and its first significant combat engagement since the Korean War (1950–53). After the 2001 terror attacks on the United States, Canada joined an international coalition to destroy the al-Qaeda terrorist network and the Taliban regime that sheltered it in Afghanistan. (See 9/11 and Canada). Although the Taliban were removed from power and the al-Qaeda network was disrupted, Canada and its allies failed to destroy either group, or to secure and stabilize Afghanistan. More than 40,000 Canadian Armed Forces members served in the 12-year campaign. The war killed 165 Canadians — 158 soldiers and 7 civilians. Many Canadian veterans of the war in Afghanistan suffer from post-traumatic stress disorder.

The Fenians were a secret society of Irish patriots who had emigrated from Ireland to the United States. Some members of this movement tried to take Canadian territory by force, so they could exchange it with Britain for Irish independence. From 1866 to 1871, the Fenians launched several small, armed attacks. Each raid was put down by government forces. Dozens were killed and wounded on both sides. The raids revealed shortfalls in the leadership, structure and training of the Canadian militia, and led to improvements in these areas. The raids also took place at a time of growing concern over the threat posed by American military and economic might. This led to increased support for Confederation.

The 1944 Battle of Normandy — from the D-Day landings on 6 June through to the encirclement of the German army at Falaise on 21 August — was one of the pivotal events of the Second World War and the scene of some of Canada's greatest feats of arms. Canadian sailors, soldiers and airmen played a critical role in the Allied invasion of Normandy, also called Operation Overlord, beginning the bloody campaign to liberate Western Europe from Nazi occupation. Nearly 150,000 Allied troops landed or parachuted into the invasion area on D-Day, including 14,000 Canadians at Juno Beach. The Royal Canadian Navy contributed 110 ships and 10,000 sailors and the RCAF contributed 15 fighter and fighter-bomber squadrons to the assault. Total Allied casualties on D-Day reached more than 10,000, including 1,074 Canadians, of whom 359 were killed. By the end of the Battle of Normandy, the Allies had suffered 209,000 casualties, including more than 18,700 Canadians. Over 5,000 Canadian soldiers died.

​The history of the Canadian Army parallels that of Canada itself. What started as a small Confederation-era militia was built into a respected force of mostly citizen soldiers for the First and Second World Wars.

The Canadian War Museum, whose roots travel back to 1880, was the first national history museum, but it was also one of the most neglected of federal institutions. Its usual fate was pedestrian quarters, meagre financial resources, and a miniscule staff.

The Oka Crisis, also known as the Kanesatake Resistance or the Mohawk Resistance at Kanesatake, was a 78-day standoff (11 July–26 September 1990) between Mohawk protesters, Quebec police, the RCMP and the Canadian Army. It took place in the community of Kanesatake, near the Town of Oka, on the north shore of Montreal. Related protests and violence occurred in the Kahnawake reserve, to the south of Montreal. The crisis was sparked by the proposed expansion of a golf course and the development of townhouses on disputed land in Kanesatake that included a Mohawk burial ground. Tensions were high, particularly after the death of Corporal Marcel Lemay, a Sûreté du Québec police officer. Eventually, the army was called in and the protest ended. The golf course expansion was cancelled and the land was purchased by the federal government. However, it did not establish the land as a reserve, and there has since been no organized transfer of the land to the Mohawks of Kanesatake.

The Seven Years’ War (1756–63) was the first global war. In North America, Britain and France fought each other with the help of Indigenous allies. At the end of the war, France gave Canada (Quebec) and Ile Royale (Cape Breton) to Britain, among other territories. This is the reason that Canada has a British monarch but three founding peoples — French, British and Indigenous.

(This article is a plain-language summary of the Seven Years’ War. If you are interested in reading about this topic in more depth, please see our full-length entry Seven Years’ War.)