Out of the fog of revolution in Iran emerges plain evidence that the regime of the Ayatollah Khomeini has trouble maintaining internal order and external security. The United States, accordingly, needs to be extremely wary in dealing with the regime.

So word that Washington is selling arms to Tehran as well as heating oil raises deep questions. Has the Carter administration developed a policy for Iran? Or is it still merely following the road to perdition traced by the line of least resistance?

What has been happening in Iran for several months now is slow fragmentation. The coming-apart begins in Tehran. The ayatollah rules chiefly through the agency of the Pasdaran, an Islamic force of bully boys recruited from the slums of south Tehran.

But revolutionary groups, including some tied to Russia, are now going into the streets to challenge the Pasdaran. Secular elements are demonstrating against the press censorship and harsh Islamic laws imposed by the ayatollah.

Decomposition extends to the corners of what has never been an entirely cohesive Persian empire. The revolt of the Kurds on Iran's western border with Iraq presents only the most salient sign of dissidence.

There is also trouble with the predominatly Arab population of the oil fields around Khuzestan at the head of the Persian Gulf, with the Azerbaijanis around Tabiz in the north, with the Baluchis on the frontier with Pakistan and Afghanistan in the east.

Already the trouble in Iran has spilled across the border with Iraq and brought a kind of preemptive coup presaging further trouble in Baghdad. Self-assertion by the Baluchis could easily rip Pakistan and Afghanistan apart. Moreover, the radical nature of the aytollah's regime, as well as its inability to police the Persian Gulf after the fashion of the shah, causes leaders all along the gulf to feel the back of their necks.

In these dicey conditions, even security at the American embassy in Tehran is uncertain. Uniformed policemen have only recently replaced revolutionary guards in patrolling the embassy compound.

The American military contingent -- which has dwindled to about a dozen -- is confined to the compound. The U.S military men cannot go outside to inspect facilities built and serviced by this country, or even to consult financial files and health records.

This country's larger national interest in a steady supply of oil is obviously in danger. The flow from Iran is jeopardized by ethnic conflict, the danger of strikes and sheer mismanagement. The other suppliers in the Persian Gulf -- the Kuwaitis, Saudis and Emirates -- have become more vulnerable to outside pressures, chiefly from the Palestine Liberation organization, which demands that they cut down on oil production. Finally there is the largest security interest -- denying Russia access to Iran.

With so much at stake, there exists an undoubted case for helping the Khomeini regime. If the ayatollah went down, the chaos would probably deepen with even worse consequences for American interests. So it makes sense to provide limited amounts of heating oil, which is in short supply in Iran.

It may even make sense to sell arms to Tehran. So far, at least, only spare parts and ammunition are involved -- not the artillery and helicopters suggested in early reports. Moreover, almost all the money required is already available from deposits previously made for weapons the Iranians are now not going to buy.

The trouble is that there is no explicit rationale for being nice to the ayatollah. The United States says nothing about the treatment of American embassy personnel in Tehran; or about the secessionist movement; or about the flow of oil; or about the Islamic terrorists; or about stability in the Persian Gulf; or even anything about the American military presence in the Indian Ocean.

Unwillingness to underline this country's tangible interests suggests that the Carter administration is engaged in wishful thinking. It hopes the ayatollah will relax his missionary zeal. It hopes more power will be ceded to the provisional government of Prime Minister Mehdi Bazargan. It hopes Bazargan and his aides will turn out to be reasonable men.

But behind those hopes there is nothing. Despite all the bluster about an "arc of crisis," the Carter administration still lacks a strategy for dealing with the vortex of world politics.