Sooner or later, in all things, the moment of truth arrives. In the case of Nicaragua, it may be upon us sooner rather than later.

Possibly as early as June 26, a treaty will be presented to the Sandinista regime in Nicaragua by the so-called Contadora group of Latin American nations. No one yet knows exactly what that treaty will contain. Nor does anyone yet know whether Nicaragua will sign it, no matter what it contains.

There has been talk of a Costa Rican initiative that would in effect guarantee the continued existence of the Sandinista regime, but would also tie it to "a precise timetable leading to democracy."

Yet as principled Leninist believers in one-party rule, the Sandinistas could never agree to this condition in good faith. They made the same promise of democratization in 1979, and they would no more keep it the second time around than they did the first.

Similarly with any promises they might make to refrain from spreading their communist revolution to other countries in the region.

In ending the missile crisis of 1962, Fidel Castro agreed to such restraint as part of the deal guaranteeing the communist regime in Cuba against an invasion by the United States. Nevertheless, Cuba has been serving for a long time now as the main source of support and training for communist guerrillas throughout Central America.

Indeed, it was Castro who literally taught the Sandinistas how to steal the revolution from the democratic forces with which they had been allied in the struggle against the Somoza dictatorship (the very same forces that are now called the contras and which are now struggling in turn against the Sandinista dictatorship).

If the Sandinistas now make a Cuban-style promise of restraint, they will again follow the example of their idol and mentor Castro by honoring it more in the breach than in the observance.

And the United States? What would we do?

In the Cuban Resolution of Oct. 3, 1962, Congress declared that "the United States is determined to prevent by whatever means may be necessary, including the use of arms, the Marxist-Leninist regime in Cuba from extending, by force or the threat of force, its aggressive or subversive activities to any part of this hemisphere." President John F. Kennedy, another great speaker of brave words, also affirmed that he would not permit Cuba to be "used for the export of aggressive communist purposes."

Yet the United States did just that; it permitted Cuba to help install a communist regime in Nicaragua. If we now accept that regime, while reiterating a suitably modified version of our brave words of 1962, why should the Sandinistas or anyone else believe that we will ever act upon them?

Of course, the Sandinistas may decide not to make any promises, not even promises they have no intention of keeping. They may think that even if they torpedo the Contadora treaty, the Democrats in Congress and the liberal community in general will still protect them from the contras.

No doubt the Sandinistas realize that if they refuse to sign, the Reagan administration will probably get some military aid for the contras out of Congress. But they must also know that it will not be enough to enable the contras to win.

In that case, the Sandinistas might even prefer to have the contras there. Too weak to pose a serious threat, the contras could provide the Sandinistas with a convenient pretext for maintaining a military force large enough to intimidate Nicaragua's neighbors into the state of political submission known in the Soviet context as "Finlandization" -- with worse yet to come.

In short, whether a Contadora treaty is signed or not, there can be no stability in the region so long as the Sandinistas remain in power. Nor will the seeds of democracy which have so recently been planted in Central America be given a fair chance to take root and grow.

Where, then, does this leave us? It leaves us with a stark choice: Either we commit ourselves unambiguously to the overthrow of the Sandinista regime, or we resign ourselves to the spread throughout this hemisphere of communist regimes allied to the Soviet Union.

Committing ourselves to the overthrow of the Sandinistas does not mean that we have to invade Nicaragua. It means giving the contras the help they need to do the job on their own.

Nor does seeking the overthrow of the Sandinistas involve any violation of Nicaragua's right to self-determination. On the contrary. Jaime Chamorro, who edits La Prensa, the Nicaraguan newspaper that was in the forefront of the revolution against Somoza, and who is now a leader of the internal resistance to the Sandinistas, has said in pleading for U.S. aid to the external resistance -- that is, the contras: "Self-determination applies to peoples, not oppressive governments that do not legitimately represent the will of the people."

Speaking of Chamorro's plea, "Don't Abandon the Nicaraguan People" op-ed, April 3 , I cannot understand how anyone who opposes aid to the contras on moral grounds, as so many liberals do, could look himself in the mirror after reading this immensely poignant document. Yet many liberals have read it, and not only do they still look themselves in the mirror, they even smile contentedly at what they see there.

Which is why, if the Sandinistas sign a Contadora treaty, Congress will in all probability cut off the contras in one blow. If, on the other hand, the Sandinistas do not sign, Congress will likely keep the contras alive but deny them enough military and moral support to win.

Either way, we will have to live with the shame and disgrace of refusing to side wholeheartedly with people who are willing to fight and die to free themselves from a communist tyranny. And since this tyranny is allied with our own mortal enemy, shame and disgrace are not the only price we will have to pay for this refusal as the years go by.