This time Marshal Tito did not come to Washington - as he once did - as a confident Third World leader reading lessons in peace and equality to the horrible Big Two. On the contrary, he has been on the defensive.

In what is probably his last bow, the 85-year-old statesman has been making a world tour to ensure the security of Yugoslavia after his death. What worries him - and what ought to worry all Americans more - is the fragility of detente between Russia and the United States.

Tito's changed role reflects in part an evolution in what was called - because it stood between the American and communist blocs - the Third World. Though poor and underdeveloped, those in-between countries of the southern continents found tongue immediately after the war in a crop of renowned leaders identified with liberation and unification. Nehru of India, Nasser of Egypt and Sukarno of Indonesia were among them. So was Tito, who helped free Yugoslavia from Hitler and then successfully stood up to Stalin.

But what used to be a bipolar world fragmented when the Chinese split with the Russians. From that point forward it was impossible to define a policy simply by being nonaligned between Washington and Moscow.

Various rivalries among the Third World countries, moreover, swirled to the surface. In addition one class of Third Worlders - the oil-exporting countries - almost overnight joined the rich man's club. Thus the Third World became an entity too diverse for any single leader or program to encompass. As his great contemporaries died off one by one, Marshal Tito came more and more to stand alone.

With his international role diminished, he turned his attention to the future of his own country. In what amounted to a coup from the top, he swept aside the regional chiefs who had established themselves in Yugoslavia's six different republics. He rebuilt the party and the army in ways that would emphasize national unity.

Then early this year he set on a world tour. Before coming to the United States, he visited Russia, China and France. Next he will go to Britain. Everywhere he has been asking - and getting - the pledge that President Carter gave in support to Yugoslavia's "independence and integrity."

But why does the old man care so much about getting assurance for the future? Because the atmosphere is not all that safe for Yugoslavia, and not only because the rivalries among the basic ethnic groups in the country persist.

The great peril is the weakening of detente between the United States and Russia. More perhaps than any other leader in the world, Tito is sensitive to the underlying forces that now work to push the superpowers apart. He has seen close up the aging of President Leonid Brezhnev in Russia, and the stiffening of policy as the succession crisis begins to take shape.

At the same time he has been made keenly aware - if only by the demonstrations against Yugoslavia - of growing popular and official dislike in this country for warm dealings with communist states. Thus by far the most interesting feature of Tito's recent interview with James Reston was his complaint about hostile demonstrations by dissident Yugoslavs in this country. "Whenever I visited America as head of state," he said, "there gathered at the place where I stayed a whole bunch of Utashi and Chetniks who escaped from our country as fascists and collaborators of the fascist occupiers."

Not only is he sensitive to the forces working against relaxation of tensions in the Big Two, but he is also witness to the outcome. He has seen in his own back yard the recriminations between Americans and Russians in the recent Belgrade meeting on human rights. He knows about Russian penetration in the HOrn of Africa the fumbling American efforts to fend it off. He is fully mundful of the Soviet military build-up, and of the slow pace of the arms-control talks.

He sees, in other worlds, that the United States and the Soviet Union are both entering situations in which they have less to gain from detente and less to lose from confrontation. That makes for an unstable atmosphere, and in an unstable atmosphere Yugoslavia becomes probably the world's leading candicate for Soviet pressure.

Unfortunatly, Tito lacks the clout to exercise much influence on either Washington or Moscow. So the best hope is that he can educate President Carter and his less cautious advisers to the dangers of the gulf between Russia and the United States, which they are allowing to widen every day.