NAPOLEON SUPPOSEDLY said that an army marches on its stomach, but the relation between war and food is broader than that. War's upheavals have often sent new foods leapfrogging borders and continents. Invasions, conquests, marching armies have all spread and cross-fertilized culinary traditions to bring forth new cusines. The Crusaders invaded the Holy Land for reasons having to do with religion and European politics, but among their more tangible booty were spices to disguise the awful fare of medieval castles. Napoleon carried back the recipe for Chicken Marengo after defeating the Austrian army at the Piedmontese village of that name. And in the aftermath of more recent military adventures, Vietnamese restaurants proliferate in American cities.

But perhaps no military campaign in history has so revolutionized eating habits as the Spanish and Portuguese conquests in the New World in the 10th century.

It is quite astonishing how limited were the larders on both sides of the Atlantic before the Conquest. In the Americas there were not cattle, pigs, chicken, sheep. No oranges, lemons, limes. No wheat, barley, or rice. No sugar, no bananas. Meanwhile, Europe's kitchens made do without tomatoes, potatoes, turkey, sweet potatoes, corn, peanuts, most beans, squash, chili peppers, avocados, and chocolate. There once was an Italy without tomatoes, an Ireland without potatoes, a Switzerland without Nestle's.

The conquest spurred a worldwide diffusion of foods.Within Latin America, the flavors of the Iberian peninsula (and also of Africa) soon blended with native cooking to produce a rich and varied cuisine, some of it familiar to the North American palates, much of it exotic and full of surprises. A good portion of that cuisine has been inaccessible to North American cooks. The books available are few and limited. But this new cookbook by Elisabeth Lambert Ortiz is a fine introduction. Ortiz not only describes, clearly and succinctly, how food is prepared throughout the region; she also weaves in enough historical and anthropological detail to make this a cultural as well as a culinary excursion.

Mexican cooking, on which Ortiz wrote an earlier book, is well represented, but she has gone beyond the border food of tacos and enchiladas the word "Mexican" conjures up in most North American minds. Two intriguing dishes are mole poblano, turkey coated with chili and unsweetened chocolate, which Hernan Cortes ate at Moctezuma's court before deposing the Aztec emperor; and sopa de lima a chicken soup made with onions, peppers, tomatoes, and lime juice which is addictively delicious.

There are seafood recipes from practically every Latin American country with a coastline, including six variations on ceviche, the piquant dish of raw fish marinated in lime juice. From the highlands of the Andes, where it was first cultivated, come recipes for preparing the potato in rich and spicy pepper and walnut sauces. Meat roasted over an open fire is synonymous with the pampas of Argentina and Uruguay, and Ortiz describes both the techniques and the variety of barbecue sauces used. Brazil is a world of cooking in itself, one where contributions from Africa are integral components. Many of the typical Brazilian desserts can be traced back through the Portuguese to the Moors who occupied Spain and Portugal for 700 years. Later Africans brought as slaves to Brazil's northeastern canefields contributed their own ingredients and culinary imagination.

Again and again in these recipes it is the unexpected combinations that surprise and delight, as in a Brazilian chicken dish seasoned with lemon juice and topped with lightly fried bananas. The mixing of fruits and meats - a tradition Ortiz traces back through Iberia to the Middle East - is common throughout Latin American cooking. One of its most appealing manifestations, both visually and gustatorily, is the carbonada en zapallo, an Argentine stew combining veal, corn, pears and peaches, all baked and served in a hollowed-out pumpkin.

In addition to the chapters on main dishes, there are recipes for appetizers, sours, vegetables and salads, breads and desserts, sauces, and drinks. They range from simple salads that could be made in two minutes to the directions for that classic Brazilian masterpiece, feijoada completa, which begins: "Two days ahead of time put the pig's ears and tail into a mixing bowl...."

One omission is the easiest and tastiest salad I know of, the Chilean apio con palta which is simply alternate slices of crunchy celery and creamy avocado drizzled with lime juice or vinaigrette. But overall, this is a thorough treatment of a unique cuisine. A chapter on ingredients gives detailed information on what goes into Latin American cooking, and what sugstitutes can be used. There is also a list of stores (including three in Washington) where the ingredients can be found. CAPTION: Illustration, no caption, by Posada