Tanzania is a vast East African nation where meager natural resources, poor planning and an incurable disease borne by the tsetse fly have thwarted the egalitarian ideals of President Julius Nyerere, Africa's senior ruling leader.

As one of the poorest nations in the world, Tanzania received a huge infusion of foreign aid in the past two decades.

That aid, however, seems to have harmed, not helped, the country's agricultural economy. As world assistance dries up, Tanzanian farmers have abandoned their foreign-made tractors -- which rarely worked -- and returned to their fields with cattle-drawn plows and hand hoes. LAND

Mainland Tanzania (the country also includes the islands of Zanzibar and Pemba) is almost the size of Texas and New Mexico combined. But nearly two-thirds of the mainland is infested with the tsetse fly-borne disease for which there is no effective treatment or cure.

Clusters of people live along the long Indian Ocean coast, in the southern and Mount Kilimanjaro highlands and in the swampy plains bordering Lake Victoria. PEOPLE

In this country of 20 million, there are 120 different ethnic groups, the largest of which -- the Sukuma -- numbers only 1.2 million. One of Tanzania's major accomplishments is that many of these groups have been unified by the spread of the Swahili language and culture. The socialist government also has been successful in improving the lot of many Tanzanians. There is free universal primary education and a 70 percent literacy rate. Life expectancy, now about 52 years, has increased 10 years in the past generation. GOVERNMENT

When Britain colonized Kenya to the north in the 1890s, Germany settled what was then called Tanganyika. After World War I, Britain governed the colony but failed to invest in it and kept white colonists out. After a peaceful, 10-year-long drive, Tanganyika gained independence in 1961. Three years later, it joined with Zanzibar to become Tanzania. Nyerere, the country's first and only president, has changed many of his people from migratory herdsmen to village-based farmers. ECONOMY

Tanzania's economy, which in 1982 managed to generate a per capita gross national product of only $280 -- 14th lowest in the world -- is almost totally dependent on agriculture. In recent years, the country's farmers have been incapable of feeding the fast-growing population. In an attempt to reverse the agricultural decline, the government has increased prices paid to farmers and cut the subsidies that guaranteed cheap food for people in the cities.