This young and hopeful nation is about to mark the first anniversary of a modern miracle: the swift, orderly, relatively peaceful conversion of a racist, repressive state, Rhodesia, into a new and viable democracy, Zimbabwe.

No precedent in black Africa comes readily to mind. Few thought it could happen here. When Robert Mugabe became prime minister after his stunning electoral triumph in February 1980, even good friends of the new government doubted it could last.

Yet it has, although in recent days it has been put to the test by guerrilla followers of Joshua Nkomo, the tribal leader who lost to Mugabe in the election. The challenge, however, was put down so quickly and decisively that Mugabe now appears even more entrenched. In the showdown, Nokomo stood by the government. It bodes well for the future.

Still, there remains one unreconciled dissenter, Sen. Jesse Helms (R-N.C.), no friend of civil rights, who is now more powerful than ever as a result of the Republican victory last fall. In 1979, Helms led a determined Senate drive to lift U.S. sanctions against white-dominated Rhodesia. If it had succeeded, the election that finally brought Mugabe to power would probably never have been held.

When Helms was recently asked if there is any hope for Zimbabwe, his answer was, "We just talked Rhodesia into the garbage dump. That country's gone."

Unfortunately, Helms, as a leader of the Senate's aggressive right wing, is in a position to hamper Zimbabwe, which urgently needs U.S. cooperation and economic aid to make the most of its opportunity.

President Reagan and Secretary of State Alexander Haig seem eager to placate the implacable Helms, but eventually they will discover, as their predecessors did, that he has a foreign policy of his own and that he will pursue it even if it is at odds with the administration's.

When that happens, will Reagan and Haig stand up to Helms as Jimmy Carter did? When Carter refused to lift sanctions and recognize Rhodesia until it held a fair, democratic election, the senator launched a campaign to reverse that decision. Carter stood up to him.

Carter was often so inconsistent on other international problem that few now remember how farsighted and steadfast he was on the Rhodesian question. If he had caved in, as many expected, after the Senate overwhelmingly backed the Helms resolution (75 to 19), it's a good bet that turmoil might now prevail not only in Zimbabwe but in much of southern Africa, too.

A lot is to be learned from the tense Carter-Helms conflict, and Reagan could do worse than pay it some head. In 1979, a palpably suspect election made Bishop Muzorewa the nominal prime minister of Rhodesia, although leaving a minority of 230,000 whites in effective control of a country with over 6 million blacks.

That outcome, however, did not disturb Helms, whose hand was strengthened by the inclination of Britain's new prime minister, Margaret Thatcher, to make book with Muzorewa and his patron, Ian Smith, the longtime leader of Rhodesia's whites.

Meanwhile, the Organization of African Unity (representing 49 African states) joined nearly all the British Commonwealth nations in denouncing the Muzorewa-Smith election exercise as a "gigantic fraud."

Tanzania, speaking for the five "front-line" nations surrounding Rhodesia, warned that Anglo-American acceptance of the Muzorewa-Smith government would be "tantamount to declaring war" on black Africa. The United Nations also was opposed to lifting sanctions.

Since Carter vigorously shared that view, Thatcher and her wise foreign minister, Lord Carrington, backed off from recognizing the Murorewa regime and called for a London conference of all the warring Rhodesian factions. Despite great odds, it paid off when the rival groups surprisingly agreed to new elections under neutral auspices.

In the balloting that followed, Muzorewa got only 3 percent of the vote, while Mugabe won a large majority. As the new prime minister, he quickly dissipated fears that he would communize the country, exterminate his enemies, drive out the white minority and embrace the Soviet Union. In practice, he did just the opposite.

While Helms may still view Mugabe as a Marxist terrorist, the prime minister has staked the future of his country on cooperation with the West, while repreatedly rebuffing Soviet overtures. In fact, grudging recognition of Russia was put off until a few days ago.

Most of America's allies have already responded with timely support, but U.S. aid has been niggardly. Nonetheless, a modest U.S. contribution could still yield a large return.