CANADA is now staring at the political question that is, after all, the fundamental one: Why should diverse people try to stay united in one country? Why bother to hang together? Americans haven't thought much about it since the Civil War. But Canada's prime minister, Pierre Elliott Trudeau, explored it at length a few days ago in his year-end interviews. As is Mr. Trudeau's habit, he left nothing to understatement.

"I'm afraid that the national will to exist as a country is not very strong in Canada, that there are all kinds of centrifugal forces, economic discontents, regional discontents, social discontents which have caused the national will to weaken over the past, I'd say, few decades," he suggested. The separatist Parti Quebecois, you remember, won the election in the province of Quebec last November. But, Mr. Trudeau philosophically observed, it was better that they come to power now than a few year later. ". . . I think Canadians were growing soft in their desire to exist as a country," he said. "You know, in historical terms we were on the way to becoming one of the freest, one of the most prosperous democracies in the world. But look around. Talk to the people. Read the media. Listen to the grumblings. Canadians aren't happy with their fare . . . [T]here's not a great feeling of content in Canada, of achievement."

Curious, isn't it? A moment earlier in the same interview, Mr. Trudeau had spoken quite explicity of the chances of civil war over Quebec separatism. He himself would not try to keep Quebec in the confederation by force if Quebec's people decide "overwhelmingly" that they want out. On the other hand, he relentlessly continued, other people might feel differently and that, of course, could lead to a shooting war. At that point he offered a very sound piece of advice: "You start shooting, and you can't easily stop."

As usual, Mr. Trudeau chose an idiom calculated to irritate the maximum possible number of this listeners. But, we should judge, he was performing a substantial service to his country. He was reminding Canadians of the truly ugly implications of some of the ideas currently being bantered around. Canadian radicals and barn-burners of various denominations have titillated themselves for years with talk of resorting to arms in Quebec. Mr. Trudeau is reminding his country that words have a way of picking up a momentum of their own. Six years ago separatist terrorists kidnapped two people in Montreal and murdered one of them. The possibility of violence is not, unfortunately, purely hypothetical.

The new Quebec government is now under intense pressure. It was helped into power by the same kind of public uneasiness over the economy that has been strengthening opposition parties throughout the western world. In theory, there is no reason at all why a separatist Quebec could not maintain a stable and prosperous economy; the separatists themselves are fond of arguing from the example of Scandinavia. But the talk of violence terrifies businesses, reasonably enough. There is also a strong streak of socialism in the Parti Quebecois; it has been duly noted among the lenders and investors of the New York money market, which is now a crucial source of job-building investment in Canada. For Rene Levesque, the leader of the Parti Quebecois and now premier of the province, it is a matter of trying to satisfy his party without alienating the businessmen - and vice versa. It is not going to be peaches and cream for Mr. Levesque.

But in the end, the outcome of the separatist challenge is more likely to be determined by the skill and wisdom of the Canadian national response to it. That brings us back to Mr. Trudeau's interview. His job is to reconcile not merely the two languages, but two cultural traditions with different ideas of law and social policy. Doesn't that mean more federal decentralization? Mr. Trudeau rapped the suggestion smartly on the knuckles and sent it off to sit in the corner."My course is to keep a strong federal government," he said, "but to make Quebecers feel that they will have a role to play in that government, and that's what I will propose." Most Canadians will doubtless wait for further details before making up their minds.

Nations generally think of wars and depressions as the times of greatest testing. Certainly that is the way U.S. history is generally written. But it turns out that there is another range of hazards, more subtle and more difficult for politicians to get hold of. The next phase of Canadian history deserves careful attention as the world's clearest example of a democratic (and intelligent) government dealing with the discontents of economc growth, physical security and rising cultural discord.