CANADA HAS now taken a step closer to the tremendous decision to split itself into two countries. A cabinet minister and key ally of Prime Minister Brian Mulroney has resigned, increasing the probability that the constitutional compromise will fail. That in turn would make it more likely that the province of Quebec will secede from Canada. It's time to think carefully about American interests in this possibility of fragmentation of a country that is a neighbor, an ally and the United States's largest trading partner.

The dominant one, overshadowing all the others, is the American interest in neighbors that are prosperous, stable and at peace with themselves. The longstanding tension between Quebec, which largely speaks French, and the rest of Canada, which generally speaks English, is getting more corrosive. The question is whether secession would reduce that friction or make it worse.

In this country, the tradition runs strongly in favor of national unity. The deadliest of all American wars was fought over precisely that issue. If Canada were to break up, the United States would run the risk of constantly being caught in disputes among the fragments. Americans would have to worry about the economic damage resulting from the uncertainties that a secession would inevitably generate. But those are not unmanageable concerns, and any American attempt to influence the choice one way or the other would do more harm than good.

Americans seem, in general, to be less distressed by the prospect of Quebec's secession than they were in the early 1970s, when it first became a serious possibility. Perhaps the explanation is that the Quebec independence movement is currently in the hands of economic conservatives and businessmen who are well known in this country. Separatism today has little of its leftish shrillness of the 1970s. The one outbreak of terrorism in those early years was quickly extinguished and has never been repeated. No one really doubts that, whether it's under one government or two, Canada will continue to be a serene and wholly civilized place in which to live and work.

Secession would affect many kinds of interests, but the crucial values here are deeply subjective: how the people of Quebec want to live and how they feel about the rest of Canada. Quebec has not yet come to the point at which secession is clearly preferable to the continuation of an intractable, obsessive, destructive hostility within the present federal union. But that is the direction in which Canadian politics is moving. If it comes to that, the dominant American interest will be in an amicable and enduring resolution, as Canadians work it out, rather than in any specific formula for unity or separation.