Islamabad — It was a gentlemen's agreement, transacted in less than two hours time. Pakistan's military leader, always the perfect host, served tea and pastry to his somewhat beleaguered guest, Robert Mugabe, who smarted that he not only had the perennial problem of Joshua Nkomo, but that his white-officered Air Force was becoming more and more of an internal security threat.
By the time the tea was finished, during New Delhi's nonaligned summit in March, General Zia ul-Haq, the Pakistani leader, had effectively outbid Yugoslavia, East Germany, and Romania to take command of the Zimbabwe Air Force. Only a few ''details'' remained to be sorted out.
Pakistani Air Vice-Marshal Azim Halepota assumed his duties in Harare in August - Pakistan's first such venture into the heart of Africa.
Such casual encounters, reminiscent of medieval European courts, have catapulted Pakistan into the third world's second-largest supplier of military manpower abroad, after Cuba.
With most of its 30,000-man ''for rent'' army now shoring up shaky leaders and guarding military and oil installations in the Arab Gulf, General Zia's martial law government has reaped the all-important patronage of the royal House of Saud.
The Saudis are reportedly paying for Pakistan's Cobra helicopters and its newly acquired F-16s, and, according to Western officials, are now providing Pakistan $1 billion in economic assistance annually. They have also been instrumental in negotiations, now under way, for the possible transfer to Islamabad of a multimillion dollar, Arab armaments production scheme, which was pulled out of Egypt after the 1979 Egyptian-Israeli peace treaty.
With military missions now in 24 countries, Pakistan's is not a mercenary army in the old, swashbuckling sense. There are no advertisements in Soldier of Fortune magazine. No lucrative, individual contracts are awarded to would-be buccaneers. It is all done on a government-to-government level. If a nervous Arab prince - most usually far more concerned with his own internal security than with any external threat - wants a personal tank escort, he need only telephone the Pakistani President.
Only the Omanis and Saudi Arabia's national guard have direct recruitment programs - the Omani legacy going back to pre-independence days, when the rugged Baluchi tribesmen of the Gwadar coastline, then part of Oman, were recruited by the sheikhdom as an elitist Praetorian Guard. Today, one-third of the Omani Navy , a skeletal force of 1,500 officers and men, is from Pakistan's four provinces, particularly Baluchistan.
But the overwhelming Pakistani military presence - whether pilots, advisers, instructors, intelligence experts or anti-terrorist squads - is in Saudi Arabia, where, according to Western estimates, there are now minimally 20,000 men. In the eyes of the Saudis, they are fellow Muslims - well trained and disciplined troops - who, as opposed to Saudi Arabia's Arab neighbors, are politically reliable, disinterested in Saudi family jealousies and tribal disputes.
An additional 7,000 Pakistanis are reportedly now being dispatched, following a new agreement, signed in April with a decidedly nervous Saudi leadership.
''This time,'' acknowledged a ranking Pakistani official, ''the President agreed that the scope of the mission would be slightly enlarged, to take over certain defense functions. But,'' he added quickly, ''it will not be a combat force.''
Publicly, this has been Islamabad's posture for a number of years, but Western defense experts and Pakistani diplomats concede that the semantics of ''combat'' really translates into no external, offensive role. But, as importantly for the vulnerable leaders of the oil-rich Gulf, they are often commandos, trained in anti-terrorist activities and riot control.
Pakistanis were instrumental in November 1979, in breaking a protracted siege by Islamic militants of the Grand Mosque in Mecca. Yet, there is a certain implicit irony that Saudi Arabia's always quarrelsome princes, consolidating their positions for future power plays, are shoring up their fortunes with the Pakistan Army today.
The two divisions sent to the Kingdom fall within the purview of Defense Minister Prince Sultan. Yet Crown Prince Abdullah, next in line to the throne, whose personal power is as commander of the national guard, has recruited on a direct, nongovernmental basis, large numbers of now retired, Pakistani ex-servicemen.
Their primary function is essentially bodyguard duty and riot control, not unlike that of the Pakistan brigade in Jordan during ''Black September'' 13 years ago, when Brig. Gen. Zia ul-Haq, then a brigade commander, played a key role in suppressing the Palestinian challenge to King Hussein's Hashemite throne.
Zia returned to Pakistan in 1972, to be promoted to major general, and given command of his first armored division, as well as a medal of honor from the Jordanian throne. According to friends of Air Vice-Marshal Halepota, the highest-ranking Sindi in the Pakistani military, prospects of a comparable elevation were offered personally to Halepota by the Pakistani President, if he took the Harare post.