IT IS TIME Americans faced up to the reality that Canada as we have known it will no longer exist by 1982. This is not yet appreciated in the United States, but the signs that the 113-year-old Canadian federation is in the process of breaking apart are very clear in Canada.

Prime Minister Pierre Trudeau, who promised Quebec's voters last May that if they voted against separation he would begin revising Canada's constitution to meet their grievances, has failed to deliver on his promises. At a specially convened constitutional conference last month, his proposals to increase the federal government's powers over the provinces were rejected overwhelmingly by the provincial governments, not only by Quebec but by those in the West.

As a result, Canada now faces fragmentation on two fronts: from a resentful Quebec, which blames Trudeau for not keeping the promises he made last spring, and from oil-rich Alberta and its neighbors. These western provinces have long resented eastern domination of the country's economic life and are now determined to fight any constitutional change that threatens their control over energy resources that rival those of the Persian Gulf.

Trudeau now has the option of rewriting the constitution in the federal parliament, where his Liberal Party enjoys a large majority, bypassing the provinces. But if he did that, he would increase the likelihood of separatist sentiments in the West as well as in Quebec. Unless Trudeau finds a formula for reconciling the conflicting demands of the West, Quebec, and his desire for a strong central government, we will probably see at least three independent nations on our northern border in about two years. One would be French-speaking Quebec. The second would be energy-rich Alberta, probably joined by British Columbia with its timber and fishing industries and its Pacific ports; Saskatchewan, with both energy and grain wealth; and possibly agricultural Manitoba. Finally, there would be Ontario, the traditional economic and political power center of Canada. The three Maritime provinces in the East -- Nova Scotia, New Brunswick and Prince Edward Island -- have long been depressed areas and would have great difficulty surviving on their own. But if an alliance could be crafted with Newfoundland, which has had major discoveries of oil off its coast recently, a fourth country of the Atlantic might be formed.

All of this unraveling could start quite soon, on the heels of the breakup of the provincial first ministers' constitutional conference in Ottawa last month. Consider this potential sequence of events:

1. Quebec's separatist premier, Rene Levesque, riding the tide of resentment over the failure of Trudeau's constitutional reform effort, will be reelected, probably next spring. With his new mandate, Levesque will hold another referendum over separating Quebec from Canada, late in 1981 or early in 1982. This time he will win, declare Quebec's independence and try to work out an economic arrangement with Ontario.

2. If Quebec breaks off, Trudeau will keep his promise to resign as prime minister rather than preside over the breakup of Canada. (Trudeau has also promised leaders of his Liberal Party that he would retire from the party leadership before the next parliamentary elections no matter what happens.) Trudeau has worked for years to maintain a united, bilingual, more centralized Canada; his retirement will set off new centrifugal forces.

3. In the three western provinces, Trudeau's Liberals failed to elect a single member of parliament last February. The defeat of Joe Clark's Conservative government by the eastern-based Liberals was a clear signal to the West that they would have little voice in national politics. The next test will come over energy policy. Trudeau's government has held the domestic price of Alberta's oil down to $16.75 a barrel -- half the OPEC price. The westerners maintain that their conventionally obtained oil will run out in about 10 years and that they need higher profits now to help set up a synthetic fuels industry for the harder times ahead. Alberta will demand higher prices for its oil and threaten to shut down its wells, and Ottawa would have to declare a national emergency to compel full-scale production at the $16.75 price. Using the Quebec separation vote as a precedent, Alberta would hold a referendum on leaving the confederation. And the motion would probably pass.

None of this, of course, has to happen. Canadians have a long history of negotiation and compromises to make their confederation work. It is possible that Trudeau, or another leader not yet on the horizon, may yet win agreement on a formula for a new but greatly decentralized Canada. It would be one in which the 10 provinces would have even greater autonomy than they have exercised recently -- and provincial powers are already much greater than those now granted the individual U.S. states. But such a Canada would have many of the same characteristics as independent states. Whatever happens should become clear within the next six months. But the Canada we have known for many decades probably is near its end.

What would the breakup of Canada mean to the United States?

In foreign and defense policy terms, it would bring complications but no insoluble problems.

Canada maintains about 5,000 troops in Europe as part of the NATO defense, and it is possible that this contingent would be withdrawn if a meaningful central authority in Ottawa ceases to exist. Ontario might wish to support a smaller contingent; Quebec, it is clear from the documents published by the provincial government before the last referendum campaign, would have no interest in supporting any military force in Europe.

Canadian ships and planes also patrol the North Atlantic as part of NATO's defense. The fleet is based primarily in Nova Scotia, and if that province remains with Ontario, the status of the fleet would remain unchanged. However, if Nova Scotia joined a separate Atlantic federation, the future of the fleet and its bases would be a subject of negotiation.

A larger question is what kind of defense relationships the new Canadian states would have with the United States. Would the North American Air Defense (NORAD) Command be expanded to include them? It is unlikely that an independent Quebec would join NORAD, even though Levesque has pledged that it would honor its air defense commitments. Would Western Canada, with its small population and limited industrial base, provide funds and forces?

The likely outcome is that the new Canadian states would cooperate with the United States, but would contribute far less to North American defense than Canada does now. Only Ontario has the population and resources to sustain an independent air force and army, and its willingness to spend for continental defense will decline if its economy is damaged by the withdrawal of the West and Quebec.

It is the economic dislocations resulting from a breakup of Canada that would pose the toughest problems both for the new Canadian states and for the United States. Quebec and Ontario have long had complementary economies, and if their separation is amicable their political and business leaders probably can work out an economic association along the lines suggested by Levesque in his referendum campaign. There will be problems in trade and monetary arrangements, but these two founding provinces of Canada have had long experience in negotiating their differences.

Western Canada's withdrawal is another matter. These provinces strongly favor free trade with other countries, particularly the United States, because for years the prices they paid for manufactured goods from eastern Canada have been 30 to 50 percent higher than similar U.S. goods. Alberta and, on a lesser scale, Saskatchewan would see independence as the way to get world prices for their oil and natural gas, both from the United States and eastern Canada.

The actor in this unfolding scenario that will suffer the most is Ontario. This heartland province which contains the national capital of Ottowa, has benefited enormously from the policies of many Canadian governments that kept the prices of western raw materials low and of eastern manufactured products high. The West is determined to reverse this imbalance; if it succeeds, unemployment in Ontario and Quebec could rise significantly.

The economic effects of Canada's breakup on the United States would probably be mixed. Americans would have easier access to the energy resources and other raw materials of the western provinces, and could sell more exports there as well. But there would be a tradeoff to the east: Ontario could well decide to put up tariff walls against the American goods now flowing into it, and a possible Quebec-Ontario customs union or common market could hamper U.S. exports and investments.

There is little likelihood that any part of Canada would want to join the United States. Too much history has gone by since the War of 1812 and the Fenian raids into Canada after our Civil War to cause our northern neighbors to seek a new union. Nor is there any present economic incentive for such a move. That day is past.

The struggle for political power being waged between Canada's provinces and the central government is similar in some respects to the "states' rights" debates that took place in the United States 120 years ago. The big difference is that Canada will resolve its power struggle by negotiation, compromise and, if necessary, by a legal separation of its parts. A Civil War is not in the cards for Canada.