"THE ideology of socialism is to fight oppression," cabled Maulana Bashani to Chairman Mao Tse-tung and President Richard Nixon alike. And the fiery octogenarian leader of the until now pro-Peking NAP (National Awami Party) charged China's leaders that if they refused to protest at "the atrocities of the military junta, the world may think you are not the friend of the oppressed".

Peking's stolidity in the face of Bengali sufferings has helped ensure - perhaps as China desired - that the war in East Pakistan will be a long one. The Bengalis have suffered military setbacks along the border with West Bengal and have lost towns recently declared Mukti Fauj (liberation forces) strongholds. But, although the April 23 appeal by the self-proclaimed provisional government of Bangla Desh for international recognition has met with virtual silence, the Bengalis have made some political gains and sympathy with their cause mounts with every day they can hold out.

Time - both politically speaking and in terms of climate-is running out for Islamabad. This may be one reason for the ominous increase in accusations against India of ''infiltration'' - and for the carelessness at least of its capture of the border post of Benapole last weekend, when mortarfire reached Indian territory at Petrapole. A fullscale international conflagration is never far from this complex crisis.

Government troops were apparently trying to secure district towns and claimed early this week to have closed the border with West Bengal-although the flow of refugees into India shows little sign of abating. Significantly the Bengalis withdrew without a fight from the provisional capital of Chuadanga and have followed up with tactics suggestive of a switch to guerilla warfare.

In this they are likely to be relatively successful, particularly now the rains are beginning. The Mukti Fauj and ordinary Bengalis are reported to have hidden large numbers of boats for use when the monsoon proper begins and the Pakistani army's tanks become more of a hindrance than the German tanks before Moscow; and the Bengalis are not as backed up to the wall as the Russians were then.

In the new phase of the struggle, logistics assume critical proportions for both sides. Perhaps the government's headaches are the worse if only because each day it must airfreight on an average 50 tons of vital supplies at a cost of Rs6 million to Rs7 million. Defence experts estimate that in the present state of Pakistan's economy the government can sustain the war for no more than four to six months.

The minimum requirements of government troops in Bengal, including supplies, ammunition and fuel, are believed to be around 120 tons a day. Eight C-130 transport planes have done twice their maximum permissible flying hours in the early weeks of the fighting and will now have to be grounded for servicing. Nine commercial Boeings have also been engaged in the airlift. And the expected reinforcements from Iran and Turkey may not materialise following Washington's discouragement. Ships carry the bulk of the supplies into Bengal but there is no dock labour to unload them.

Distribution within Bengal of army supplies is equally difficult. A small fleet of Fokkers and helicopters have been bearing the brunt of this burden. This has been growing less and less reliable with fuel hard to come by and some air fields put out of commission by the liberation units. When the rains set in next month, the fleet may be as good as useless.

Estimates of casualties among government troops are so far modest-3,000 by the best count. Military experts say if this figure touches 10,000 the army's strike capability will be grievously affected. West Pakistani forces in Bengal are put at anything between 50,000 and 80,000. If the government really intends to seal the border, at least 40,000 troops will be required for this purpose alone-which highlights the hopelessness of the objective.

The liberation forces will by comparison have a better time of it during the monsoon. But they must keep supply routes from India open-and hold on to a firmly held territory as headquarters. Until this week, Faridpur in the south and Mymensingh 80 miles north of Dacca were both strongholds of the liberation forces. But Pakistan army units have reached Faridpur; and heavy fighting is going on around Mymensingh, which has suffered strafing and incendiary bombing and is being attacked by a three- pronged ground force backed by 24 Russian T-59 tanks. Army strength in the northwest, based on Rangpur, has been increased to over a division backed by armour and artillery.

The liberation forces have received a shot in the arm with the formal inauguration of the "mango grove cabinet" (in "Mujibnagar", a village now teeming with government forces). With the surfacing of an experienced politician like Tajuddin Ahmed-he was general secretary of the Awami League-the political vacuum at the top is now filled. If he succeeds in putting the organisation together again with some shrewd accommodation to sections of Maulana Bashani's followers, he could even seriously threaten the chances of Mohammed Toha's Maoist EPCP-ML (East Pakistan Communist Party -Marxist Leninist) taking political advantage of the liberation struggle. In that case, the risk China has taken in openly siding with the military junta in Islamabad may yet prove too costly.