President Reagan is getting into slippery territory when he asks the French to intervene in Chad on the ground that it is ''not our primary sphere of influence but that of France,'' and when people around the White House justify US intervention in Latin America on the ground of the Monroe Doctrine.
The Monroe Doctrine is a ''sphere of influence'' doctrine. It asserts a special US interest in what happens in the Americas and a special US right to say who may and may not try to move into that area.
But is is often forgotten that the original Monroe Doctrine was two-sided. The other side of the doctrine was that the US had no interests in European affairs, had no desire or intention of getting involved in Europe, and hence wanted Europeans to keep out of the Americas. In effect it said, ''We will keep out of Europe provided you keep out of the Americas.''
The Soviet Foreign Office would be delighted to see a revival of the Monroe Doctrine in its original meaning. Moscow thinks of Europe as being as important to its interests (in fact, it is a great deal more important) as Latin America is to the US. Russia has been invaded four times in modern history from Western Europe. President Reagan could get instant agreement from Moscow were he to say, ''I'll keep out of Europe if you will keep out of Latin America.''
But there lies precisely the reason why American diplomacy has avoided talk of ''spheres of influence'' in this century.
Matters were different when James Monroe was President of the United States. Back when he delivered his annual message to the Congress on Dec. 2, 1823, the US was made up almost entirely of people from the British Isles. There were a few French-speaking persons in Mississippi and Louisiana, some Swedes in Delaware, a few Portuguese in Rhode Island, and the ''Pennsylvania Dutch.''
But those were minor exceptions to the basic complexion of the early US population. It was overwhelmingly made up of people who had left the British Isles to get away from everything European. There were almost no Italians or Spanish or Hungarians or Slavs from Eastern Europe. There were virtually no ethnic or cultural minorities who cared about any particular part of continental Europe.
And as for those who came from the British Isles, they had just finished fighting their second war (the War of 1812) with the British crown. They regarded Britain as the traditional enemy.
In 1823 the last thing in the world most US citizens wanted was to get involved in Europe.
Times change. By now every country in the world has sent some of its people to the US. And many of them care very much about what happens to their countries of origin. The Poles, Czechs, Hungarians, Latvians, Lithuanians, and others care enormously about Soviet policy. They would like to see the US government drive the government of the Soviet Union into the Baltic Sea and liberate all the ethnic minorities living under its iron heel.
At Yalta, at the end of World War II, Winston Churchill wanted to do a deal with the Soviets for southern Europe. He proposed that Moscow would have Hungary and Romania in its sphere of influence. Britain would get Greece and Turkey, and Yugoslavia was to be shared on a ''fifty-fifty basis.'' The US delegation objected vehemently. It had no intention of laying itself open to the charge of handing Romanians, Hungarians, and Yugo-slavs over to Moscow. The US never overtly recognized any Soviet sphere of influence anywhere. The Helsinki ''final act'' has been interpreted by some as a tacit recognition of such a Soviet sphere in Eastern Europe, but this has always been denied by US government officials.
The reason is obvious. To recognize a special sphere of influence could be self-denying to the US which has interests everywhere. Also, if the US asserts a special ''sphere of influence'' for itself on grounds of proximity (Latin America), it by so doing provides the Soviets with an argument for asserting spheres of its own on grounds of proximity.
Washington is not willing to declare its noninterest in Eastern Europe, or in Turkey, or in Afghanistan, or in Pakistan, or in China just because those territories lie next door to the Soviet Union.
The sphere of influence game is a risky one, particularly for the US which is made up not of a homogeneous population (like Japan or France or Italy) but of peoples from every country in the entire world. It is by its very nature interested everywhere. It had best leave ''spheres of influence'' to others.