PARIS -- Europe is gearing up to fight a trade war with Japan. But the newly confident Europeans risk building economic Maginot lines, planning their campaign with outdated tactics from battles past.

The Japanese, as usual, are not thinking of yesterday or in quarterly or semiannual terms. The gaze from Tokyo is fixed in the far distance. After intensive analysis of the arrival of the post-postwar era in Europe, Japan positions itself astride the new currents of political and economic change for the 1990s.

If trade war does break out between their allies, Americans will be tempted to take sides against the Japanese out of frustration with trade deficits or out of prejudice. But that will be a self-wounding reflex if Europe turns protectionist and inward-looking. In those circumstances, America's interests could lie with Japan.

France, Italy, West Germany and other continental nations of the European Community see their domestic markets for autos, consumer electronics and other areas in which Japan decimated American manufacturers in the 1980s as the next major battlefield in world trade. Europe's weapons will be ''voluntary'' trade restraints negotiated with Japanese exporters and other devices to protect markets against the Japanese ''invasion'' expected here, since American markets are reaching saturation.

If Japan ''attacks'' here, European officials warn, the single EC market of 1992 will become a Fortress Europe as far as Japan is concerned. It is difficult to see how a selective fence would work to keep Japanese goods out but let American products in; but the intent to erect such a fence is there.

For Japan, cars and VCRs represent yesterday's battles. Voluntary restraints magnify Japanese profit margins by restricting the supply of highly desirable products. Moreover, manufacturing grows less important to Japan each day. The current issue of The Economist magazine points out that Japan saves $300 million more than it invests every day of the year. The Japanese establishment will use this constantly increasing financial power to reshape the international trade environment in Tokyo's favor.

The intriguing behavior of Japan at the Houston economic summit earlier this month and the pattern of Japanese investments in Asia and Europe show that Japan has embarked on a campaign to prevent the global trade and financial system from splitting into regional trading blocs. As part of that effort, Japan is shifting emphasis from treating America and Britain as marketplaces to wooing America and Britain into becoming strategic partners in the trade conflicts of the 1990s.

At the Group of Seven summit in Houston, Japan found itself allied on the hot-button issues with Britain. Margaret Thatcher and Toshiki Kaifu both dug in their heels on extending economic aid to a collapsing Soviet economy, despite West German and French importuning. And Thatcher, worried about Hong Kong, joined Kaifu in the private sessions in favoring eased sanctions on China.

George Bush drifted on both issues. But he seemed to tilt quietly toward the British and Japanese prime ministers and away from the continentals, who vexed the president by pushing hard as a unified bloc on Soviet aid, agriculture and other trade issues.

''Going from the NATO summit in London to the G-7 summit in Houston was like going from the past into the future,'' said a participant in both gatherings. ''At NATO, there are established patterns of behavior that mean American ideas always get a full hearing. In the G-7, the Europeans presented the United States and Japan with decisions they had already made as the EC.''

German reunification and the collapse of the Soviet empire add to pressures working to create a European regional trading bloc that would go its own way. This would reinforce similar dynamics in the Americas and in Asia -- resulting in dollar, German mark and yen zones of trade and finance.

At Houston it became apparent for the first time that Japan is concerned about the long-term economic implications of the emerging German-Soviet partnership. Kaifu's refusal to consider aid to the Soviets was based primarily on Japan's claim to the Kurile Islands and skepticism that the Soviets could use the aid effectively. But there was also an element of not wanting to go along with a plan that might help entrench a Bonn-Moscow directorate from the Rhine to the Urals.

Once December elections and unification are out of the way, Germany will quickly turn its attention to its new leadership role in Europe. It is likely that a solidly reelected Chancellor Helmut Kohl will make a dramatic proposal for a quick and binding European political federation to accompany economic and monetary union. The proposal will be designed to take away the breath, and maneuvering room, of the British and the French.

Japan's heavy investment in Britain and Kaifu's diplomatic complicity with Thatcher in Houston may help disrupt the protectionist front that some in Europe want. Although they are late converts, the Japanese now sound genuine in their desire for alliances with the free-trade forces that govern in Washington and London. It would be in their interest, of course.

But it may also turn out to be in America's interest.