The Cold War may be ancient history in the United States, but it is still the reason for being for many of Latin America's rightist military governments.

Seeing themselves as the last lines of defense against an international Communist conspiracy, they feel angry and betrayed by what they consider U.S. capitulation to the deadly charms of coexistence with the Soviet Union and its western hemisphere surrogate, Cuba.

In speeches this week to the Organization of American States, Argentina, Chile and Uruguay have hinted that Washington's preoccupation with human rights in their countries may be evidence that President Carter is too blind to see the threat, or may even be a leftist sympathizer.

This perception and the debate over the legitimacy of one country's criticizing what goes on in another has caused nearly every delegation attending this year's OAS General Assembly to take sides on the U.S. human-rights campaign. It threatens to fracture any effectiveness the organization may have.

Chilean Foreign Minister Patricio Carvajal expressed in a speech the rightist military government's feelings of betrayal.

"Instead of helping us in out struggle against this danger that is growing on our continent," he said, the United States "has restricted or suspended military American countries who have made the decision to defend themselves."

He was expressing the view of many military rulers who see themselves as the only ones capable of combating leftist subversion in their countries. Asking them to admit that leftist opposition does not require strict military control is seen as asking them to give up power.

Many of the military officers now ruling Latin countries were trained on U.S. military bases when the expansion of Soviet communism was considered the ultimate global threat.

Describing the Communist threat in his country, Carvajal attributed it in part to the U.S. policy of detente with the Soviet Union. While lessening the threat of nuclear war, detente "left the hands of the Kremlin free to attack the developing world with ideological penetration and terrorist subversion," he said.

The fact that U.S. relations with the Soviet Union have actually worsened under the Carter administration - and disputes over arms control, trade and even human rights - has little meaning to Latin America's rightist military rulers.

They see the U.S. refusal to assist them in the war against leftist opposition and Carter's policy of detente as a way of giving aid and comfort ot the enemy.

Representatives of the military governments say the United States has responded to their attempts at patriotism by allowing them to be called murderers, tortures and demagogues - allegations that they refuse to believe are motivated by selfless concern for human rights.

Arguing that human-rights criticicisms are selective, immoral and illegal, Uruguayan Foreign Minister Alejandro Rovira charged that they depend largely on "strategic and economic interests of a given country at a given time."