Senior American diplomatic believe a Soviet military thrust into Poland has become inevitable. They foresee a catastrophe that will cast dark shadows over Soviet relations with this country and the rest of the world.

But for tactical reasons they are muting that extremely somber view. They do not want to deaden reaction by advance publicity. They do not want anybody -- particularly our allies -- to shrug off the Soviet move as someone in the cards all along, something to be expected, business as usual.

"Near anarchy" is the phrase used in the State Department to describe conditions in Poland. These are the elements of the situation as seen by the highest officials:

The Polish workers are out of control. The Polish farmers are out of control. The Polish students are out of control. Even the rank and file of the Communist Party is exceedingly restive.

Institutions for order and disipline have lost their grip. The trade union leadership of Lech Walesa would like to calm matters. But it has no authority over the vast number of workers and farmers now rising in spontaneous protest. The Catholic Church believes in coexistence with the Communist authorities. But it cannot intervene against students who resist the compulsory teaching of Marxism.

The Communist Party leadership seeks to avoid confrontation with any of the constituent groups in the country -- the more so as it might serve as an excuse for Soviet intervention. Accordingly, the party leadership has been making concession after concession after concession.

It gave on wages in the first showdown with the unions last summer. Then on hours. Then on registering the unions officially. It also gave on ousting the former party secretary, Edward Gierek, and many of his associates. It gave in accepting more access to the media for the trade unions. And for the church. It gave, most recently, in ousting Jozef Pinkowski as prime minister and replacing him with a former defense minister, Gen. Wojciech Jaruzelski.

Jaruzelski is known as a modernate nationalist. He is Soviet-trained and thought to have the backing of Moscow. As defense minister, he refused to use force against the dissident workers. He has no political ambitions and apparently turned off a move to make him party secretary last summer. His appointment to be prime minister is seen here as a "smart move." But perhaps not smart enough. The State Department likens the tactics of the Polish party to the peeling of an onion. It has taken off one layer after another. Now it's very close to the core.

Soviet authorities, it is believed here, have gone along with the onion tactics. The Russians are thought to have little stomach for a full-scale invasion. They understand well the toll that would be taken in military terms alone. Also the economic burden that would devolve upon Moscow. Finally, they realize the Soviet Union would receive a black eye all over the world.

If possible, it is believed here, the Russians would prefer Poland to do the heavy work. They would like Polish soldiers and Polish police to apply force. They would like Polish political leaders to give the orders. If intervention is required, Moscow would prefer to be invited in by the authorities in Warsaw.

Accordingly, though unhappy about the evolution of events, the Russians have accepted the policy of concession so far. That relatively tolerant attitude is expected to last at least through the Congress of the Soviet Communist Party beginning in Moscow on Feb. 23. The State Department believes strongly the Russians do not want the party congress messed up by an invasion of Poland. But thereafter, it is thought, the Russians too will be reaching the core of the onion.

If the Soviets do invade, the Reagan administration expects a world crisis. Secretary of State Alexander Haig would fly immediately to Europe to confer with the heads of allied governments. A total embargo on economic dealing with the Soviets would be applied. New Western defense efforts would be spurred forward. Political dealings with the Russians, particularly in arms control, would be suspended. At the United Nations, and in other international forums, there would be moves for harsh condemnation.

Some hard-liners may even welcome a Soviet move on Poland as an opportunity -- a chance to alert this country and its allies to the challenge posed by the Russians. But that is not the view at the senior levels of the State Department. Here the prospect of a Soviet move against Poland is regarded as a foretaste of a genuine cataclysm. One has the sense that there is brewing a storm that could shatter allegiances and shift the grouping of nations all over the globe. If there is a hope, it is that the expression of these views in private diplomatic exchanges will deter the Kremlin from taking the plunge.