El Salvador, the tiniest, most overcrowded and among the poorest of Latin American countries, is the quintessential "banana republic," a nation that is almost a caricature of itself. It is controlled largely by military officers -- an unsavory collection of overfed, sunglassed men who exude unsophisticated cruelty. Its civilian leaders talk of the need to control communist subversion, and declare themselves beloved by a population that had no say in choosing them.

Most of its people are malnourished peasants who live in mud huts and survive on what they can scratch from the resource-poor land. There are even guerrillas -- young, leftist, anti-American and as confident as their enemies in the government of their overwhelming popular support.

More than 13,000 have died there in the past year, most of them through political violence. U.S. military aid to the Salvadoran government soared under the Carter administration. Under President Reagan, whose stated commitment to stopping Central American dominoes from falling to the communists is untempered by human rights considerations, American involvement there is likely to increase massively.

Thus the PBS broadcast of "El Salvador: Another Vietnam?," scheduled tonight at 10 on Channel 26, could not come at a better time. Billed as the "first American documentary to be filmed in El Salvador during the current political crisis and wave of violence," the hour-long production is designed to answer, before a similar tragedy occurs in Central America, the kinds of questions Americans began asking too late about our involvement in Vietnam.

Unfortunately, the WNET-New York production, filmed on scene in El Salvador just last month, is likely to raise more questions for those uninitiated in Central American reality than it answers.

The conclusions the documentary reaches are clear, and many who have followed the Salvadoran situation closely over the past year will agree with them. The current Salvadoran government is unrepresentative; the military, in its unrelenting zeal to liquidate the roots of a perceived Cuban-authored communist conspiracy to overthrow it, is responsible for most of the civilian deaths that have occurred, and the United States, consciously or unconsciously, has misunderstood the situation and runs the grave risk of becoming embroiled in another Vietnam.

"El Salvador: Another Vietnam?" includes much eloquent footage of poverty-stricken refugees driven from their homes by the military. The voice of Ita Ford, an American nun working with the Salvadoran poor who was killed, allegedly with official complicity, shortly after this recording was made, is a testament to both the cruelty of the armed forces and the fear of the peasants.

But where are the guerrillas to speak for themselves? Without a true picture, or even a cursory exploration, of the ties they may or may not have to Cuba, of the extent of their support among the populations, there is little basis on which to judge Defense Minister Guillermo Garcia's claims that it is the guerrillas who are causing all the problems.

The documentary does not address the longtime hold over El Salvador of a landed and extremely wealthy economic elite who are believed to be supporting, and funding, the ruling military right. It presents no details of the amounts or types of U.S. military aid currently going to El Salvador.

"El Salvador: Another Vietnam?" is an initial attempt to address what will be a vital question to the U.S. public in the near future. It is worth watching, but it does not provide the kind of information Americans will need to come to an intelligent answer.