President Reagan's recent statement that Chad "is not our primary sphere of influence" but is "that of France" may serve as a reason for not directly entering the warfare in that former French colony, but it also opens up some much larger questions involving both the United States and the Soviet Union.

"Sphere of influence," like "balance of power," is one of those old European terms long shunned in American diplomatic usage, however much both have been practiced in reality.

The president may prefer to leave Chad's problems to France, but he certainly is not ignoring either "spheres of influence" or the "balance of power" when it comes to Central America, specifically to Nicaragua and El Salvador. Indeed, both are central to his policy there, though he has yet to say so out loud.

A bit of history explains much.

The United States was founded and developed by those who came here from Europe, some for adventure or gain but millions also to escape homelands wracked by wars and revolutions. They thought those disasters had been spawned by cynical governments playing balance-of-power and sphere-of-influence games with human lives and they resolved that in the New World there would be none of that. Isolationism, they thought, was the American way to escape such Old World intrigues.

After World War I, Woodrow Wilson sought to create a League of Nations as a substitute for those European ways of managing relations among nation-states. But the U.S. Senate rejected the League, and we returned to isolationism, at least in theory.

After World War II, Franklin D. Roosevelt tried a new substitute, the United Nations. When he came back from the 1945 Yalta Conference with Stalin and Churchill, FDR told a joint session of Congress that the outcome of that conference "ought to spell the end of the system of unilateral actions, the exclusive alliances, the spheres of influence, the balance of power, and all the other expedients that have been tried for centuries--and have always failed."

This time we did join a world organization but it didn't work out the way FDR foresaw. Instead, the two new superpowers ever since have played the old Realpolitik games. For one thing, the defeat of Hitler left the Red Army in control of Eastern Europe, creating a Soviet sphere of influence from the Baltic to the Adriatic cut off from the rest of Europe by what Churchill called an Iron Curtain. "Chip" Bohlen, Roosevelt's interpreter at Yalta and later ambassador to Moscow, frequently rejected the charge, often hurled by Republicans at Democrats in the Cold War era, that FDR had, at least tacitly, agreed to that Russian sphere of influence. Reagan may encourage rebellion in Poland, for example, in the form of the Solidarity trade union, but like every president since FDR he recognizes that to attempt to change the Eastern European status by force would bring on World War III.

President Eisenhower's secretary of state, John Foster Dulles, boasted that his diplomacy involved "the ability to get to the verge without getting into war." In short, brinkmanship. And he added that "if you try to run away from it, if you are scared to go to the brink, you are lost. . . ." That his brinkmanship was effective later was attested to by Nikita Khrushchev; that Soviet leader called it "barefaced atomic blackmail" but he acknowledged that "it had to be reckoned with" at a time of vast American superiority in nuclear arms.

Khrushchev's attempt to sneak nuclear missiles into Cuba was, most importantly, an effort to tip that unfavorable balance of power in his favor. President Kennedy's response became the hair-raising 1962 Cuban missile crisis; Khrushchev backed down and took his weapons home.

But Fidel Castro's Cuba remained a communist satellite, armed and succored by the U.S.S.R. Thus the current Central American problem, in Reagan's view, is the result of Castro's aid in the form of both arms and advisers to the Sandinista regime in Nicaragua and to rebels in El Salvador and elsewhere in the area. All this, as the president looks at it, is being manipulated from Moscow, center of the "evil empire," and it constitutes an unacceptable Soviet effort to take over a part of the American sphere of influence--Latin America--and thus alter the East-West balance of power.

The American contention that this hemisphere is in our sphere dates from the creation of the Monroe Doctrine back in 1823, and conservatives such as columnist William F. Buckley Jr. again are calling for its resurrection and enforcement.

Reagan has acknowledged that widespread poverty and abuse of human rights may have provided fertile ground for communist subversion and intervention in Central America. But he is operating on the thesis that the solution is to force out Soviet-Cuban power and influence. Hence the current flow of American arms and advisers and the massive show of naval and troop strength. In his recent letter to the four Latin American presidents of the Contadora Group, Reagan declared that "the conflict in Central America must be removed from the context of an East-West confrontation." And how? By withdrawal of "all foreign military and security advisers" and by putting an end to the inflow of "offensive armaments." Presumably, then, any subsequent civil warfare would pose only minor problems for the United States.

Reagan now has named Henry Kissinger to chair his Central America policy panel. One reason many Americans dislike the former secretary of state is that he seems to embody Old World machinations such as spheres of influence and the bpower. In the second volume of his memoirs, he wrote that "we in the Nixon Administration felt that our challeAmerican people in the requirements of the balance of power," and he lamented "how little domestic support" he In part, he joined in d,etente with the Soviet Union to balance American weakness growing out of the disastrous intervention in Vietnam. But those policies also reflected his study of the European Congress of Vienna, which led him long ago to write so favorably of "a general equilibrium"--a balance of power--which "makes it risky for one power or group of powers to attempt to impose their will on the remainder." It thus is not difficult to see how he views the problem of Central America.

Reagan's current emulation of Teddy Roosevelt's admonition to "carry a big stick"--even though he is not practicing the other part of TR's rule, to "speak softly" --does force the Soviet Union to face up to some important policy evaluations, perhaps to make painful choices. For example: is it important enough for the Kremlin to continue to make inroads into the American sphere of influence in this hemisphere to risk Reagan's non-cooperation in coming to arms control agreements that could either prevent installation of new American missiles in West Germany or lower the dangerous level of rival intercontinental missiles, or both?

In short, Ronald Reagan is playing old-fashioned, European-style power politics involving both the superpowers' spheres of influence and the balance of power itself between the United States and the Soviet Union. Whether this is wise or ruinous, moral or immoral, is something else again. But such games seem inescapable as long as this is a world of suspicious, rival nation-states.