After a year spent trembling on the very brink of a violent national crisis - one that could well threaten the unity of the country itself - Pakistan is about to face up to the most important decision of its three-decade history.

Should former prime minister Zulfiqar Ali Bhutto be hanged for his conviction in a political assassination plot? Or should the military leaders, assuming Bhuttto's appeal fails, spare his life?

To most observers it appears that the man who has to decide, the chief martial law administrator Maj. Gen Mohammed Ziaul-Haq, faces a problem that is truly intractable.

"He's damned if he does, and he's dead if he doesn't," said a diplomat in Pakistan's capital, Islamabad.

Two days ago, however, there occurred a small event that could well provide the beginnings of an answer to zia's dilemma.

It was, to most outside the country, an event of apparently minimal import. A middel-aged Islamic leader, Maulana Kauser Niazi, indicated that he would join Zia's effort to form a "national government ."

Other Mullahs and Maulanas - Islamic priests - have agreed to lend their weight to Zia's proposal with little impact. Why, then, the ripple of excitement over Kauser Niazi?

The principal answer is that Niazi was once regarded as a key Bhutto Man - a former religious affairs minister, in fact, in Bhutto's last Cabinet. Over the past few months, though, he has been moving away from Bhutto. Indeed it would be accurate to say that the Pakistan People's Party, which Bhutto helped form, on the verge of a split thanks to Niazi.

One faction is composed of the Bhutto family and their admirers; the others is a small band dissidents led by Niazi. Until this week both wings of the Pakistan People's Party had ignored appeals to cooperate from Gen. Zia. He, in fact, had arrested so many party officials aligned with Bhutto that he was accused of trying to break the party, and both factions, loyal to a common purpose, had spurned him.

Now, whether by Zia's doing or by some internal revolt, the party does seem divided . The Niazi faction, small but growing, is now willing to cooperate with the military. The Bhutto faction is now isolated.

In theory at least Zia should find Bhutto - his major problem - easier to tackle.

Until now, had Zia hanged Bhutto, he would face massive demonstrations in the streets - perhaps amounting to civil war - from the millions who still support the former prime minister,

Had he commuted the death, sentence and sent Bhutto to jail or to exile, Zia faced the probable retur to power one day of the Peoples Party - and Bhutto. What might the generals expect from the man they had so humilated.

"Zia and his felloe generals wouldn't have a chance," said the diplomat in Pakistan. "They'd have to be on the first plan out, and be gone forever.

Yet both these scenarios postulated that support for Bhutto remains gigantic. This is where Niazi's move requires crucial significance.

Until this week, a stern policy of arrests, floggings, jailings and public executions, have kept Bhutto's supporters sufficiently crowd.

Now, though, thanks to the persuasive abilities of Zia - and perhaps because of some loibbying by friendly Arab diplomats, the Pakistan Peoples' Party just might, be on the verge of a breakup. If thousands follow Niazo and claim, as some have already done, that Bhutto's trial showed him to be a crook, and worse, then there might be relatively little opposition to his execution - or little chance of his return to power if the death sentence is lifted.

Zia's problems, therefore, seem a little easier.

Yet, into this complicated mixture must be stirred the innumerable message of protest on Bhutto's behalf from other governments. So strident were some of them that Pakistan sent out its ambassadors to launch a blitz on offending editors and commentators throughout the world. The court verdict and sentence were fair, they said, Please keep quiet for the time being, they asked. We might well accuse you of interfering with our internal affairs, they warned.

The tide of protest became a tricke. The sultan of Osman was one of the last to demand clemency, before the world shut up. Yet the big ones, the countries to whom Pakistan must listen, had already spoken. China Pakistan's armorer; Iran, its most friendly neighbor, and Saudi Arabia, its financial benefactor had all urged Zia to spare Bhutto's life. Given the power and influence these three have in Islamabad it is unlikely that they will keep quiet.

So, will Bhutto die?

Two weeks ago, when streets protests had seemed to have been blunted by repressive tactics, and when Zia was making remarks hinting that he would never spare the man, it is looked as though he would.

Today, with influential friends protesting, with Bhutto's party dividing, the fate of the former prime minister appears rather less certain.

If Bhutto dies, he will be the first elected leader of recent times to be executed after a criminal trial.