If the United States is to go to war in the Persian Gulf, and war now seems unavoidable, Americans should understand the one reason that justifies so grave a course of action. The argument may be stated in a few words: If not now, when?

This was the unanswerable question that finally led to Saturday's 52-47 vote in the Senate to free the president's hands. Given a choice between the possibility of prompt action on the one hand, and the prospect of indefinite delay on the other, the Senate chose the former. It was a choice both wise and terrible.Wise, because the vote reflected the experience of ages. Terrible, because hundreds, perhaps thousands of young men will die in the bloody fighting that apparently lies ahead.

Since this crisis first developed on Aug. 2, the president has advanced half a dozen explanations for his forceful intervention. We must act, he said:

- To punish aggression in violation of international law. But the argument had a hollow ring. The United States did not rush troops to Afghanistan to repel what amounted to Soviet invasion. We ourselves took action in Grenada and Panama with only the shakiest support in "international law."

- To restore the emir of Kuwait to power. But this was a goal with no appeal to anyone save the emir himself.

- To support the United Nations. But inconsistency again has reared its mocking head. We have not supported the U.N. General Assembly in the matter of Israel.

- To preserve the world's access to Middle Eastern oil at a reasonable price. But this purpose smacked of materialism, and in any event, it was truthfully argued that a shortfall in supplies of oil could be made up from other sources.

- To bring about a "new world order" in the Middle East, putting an end to recurring hostilities involving Muslims, Christians and Jews. But the region has known 4,000 years of upheaval and warfare, and little evidence supports the dream of stability now.

These were the major arguments advanced in support of the coalition against Iraq. Each of the reasons had some measure of validity, but collectively they were not enough. The balance tipped with the weight of evidence that Saddam Hussein is well along in the manufacture of a deliverable atomic bomb. The prospect is intolerable. It cannot be ignored.

On the House side of the Capitol, eloquent debate preceded the vote of 250-183 to support the president, but the House action was anticlimactic. The Senate provided the principal field of battle, and on the floor both sides distinguished themselves. It was the Senate's finest hour in many years.

One after another, senators spoke to the issue: Majority Leader George Mitchell of Maine pleaded for time to let sanctions work. Sam Nunn of Georgia warned of unpredictable consequences. Virginia's John Warner, supporting the president, worried about the message that would be sent if the president were rebuffed. In three days of somber debate, this observer detected few overtones of political partisanship. The Senate can behave disgracefully, irresponsibly, indefensibly - but here the Senate performed superbly.

"These are the times that try men's souls," said Tom Paine a long time ago. "The summer soldier and the sunshine patriot will in this crisis shrink from the service of their country; but he that stands it now deserves the love and thanks of man and woman. Tyranny, like hell, is not easily conquered."

Paine was pamphleteering in December 1776, but his message has lost none of its clarion appeal. Let us pray for our men and women in arms, and may Saddam be damned.