Canada was not consulted as to whether or not they would go to war in World War I, as they were still part, more or less, of the British Empire and felt honor bound to defend the Mother Country. Their sacrifice was significant, and more than most people realize, as they lost 67,000 killed and 173,000 wounded, in a country with less than 20 million people total. While on the winning side, I would say such a sacrifice in blood and treasure weakened both Canada and Britain.
World War II was a slightly different situation, as Canada was an independent nation as of 1931, and entered the war as a member of the British Commonwealth considerably later than Britain and France did, and by a vote of Parliament. Their contribution to the war effort was no less significant, as over a million Canadians served in the war, their military participated in the D-Day landings, and their Navy and Air Force were crucial to the Allied supply effort. The threat was more grave, in my opinion, in World War II, since actual occupation of England was a possibility, and loss of the shipping lanes in the Atlantic and Pacific even more likely. While her sacrifice was great, the peace and security Canada bought with those losses was real and lasting.
Canada's contributions to either the Cold War or the War on Terror have been much more limited in scope, meeting her obligations to the NATO treaty alliance but maintaining a very limited armed forces. Blessed by geography and a relatively sustainable economy and energy supply, Canada has faced less security threats in the modern day, and I think, has felt less obligation to engage threats to her allies. Canada has thus emerged as one of the more stable economies even in the face of world recession.
The costs and benefits of war are always debatable. During WWI government debt rose from$463 (1913) to $2.46 billion (1918)(http://www.thecanadianencyclopedia.com). On the other hand, exports in terms of natural resources rose, and then later as the war progressed in munitions. An issue over conscription had negative effects. French Canadians strongly opposed conscription until 1918, and this had a divisive effect on the country. On the other hand, Canada's overall military contribution established Canada as a power in its own right, to the extent it had separate billing in the League of Nations.
In the run up to WWII, Canada supported the policy of appeasement but when war was declared, the Canadian Parliament accepted sending troops overseas. Once again, Canada's forces fought gallantly. In economic terms, Canada's industries grew even larger than they had in WWI, producing more than just munitions. As a result, Canada's industrial base was strengthened as a result of the war in much the same way as the United States.
The Cold War started with Canada as a supporting player in the western alliance. Canadian politicians opposed the spread of communism and was a founder member of NATO. Canada's strategic position meant that NORAD and other American defence agencies relied on Canada to provide early warning bases. Far from being a subordinate partner, the Cold War found Canada in a position which often opposed its southern neighbor. For example, Canada's opposition to the Vietnam war and American support of dictatorships in South America.
stronger ... GO CANADA!