TO THE NEWLY signed agreement between Ian Smith and "internal" nationalists to establish a black-ruled Zimbabwe (it's now Rhodesia) by year's end, we offer two cheers. The agreement includes guarantees of black rights and white rights in a mix satisfactory at least to those in Salisbury. It promises elections in which the government would have strong incentive to see that most blacks took part. Are there flaws in the new plan? It would be presumptuous for Americans to substitute their judgment for a Zimbabwean mojority's freely offered judgment of what form multiracialism and democracy should take in another country, especially a country at war.

A country at war - that's the problem. We must withhold that third and last cheer because the new agreement cannot of itself halt the guerrillas. Their pressure produced the Salisbury compromise whose fruits others, not they, are trying to reap. One does not have to approve of their method (armed struggle) or patronage (Russia, China) or ideology (tending leftwards) to realize that no plan that fails to deal with them is complete. To bless unreservedly an internal settlement without figuring in the guerrillas is cheap.

The guerrialls face the prospect of opposing an elected black government that will be protected by the reconstituted Smith armed forces, and that is attracting a certain international support. On that basis the more optimistic internal people feel that the guerrillas will be beaten back or will fade away.But it's a gamble. The new agreement, in its general principles and in the interim powers it accords Mr. Smith and his armed forces, has what guerrillas see as a strong pro-Smith tilt, and that seems to be confirming their view that it's too early to consider cashing in military chips for political coin. They'll fight on.

It has been the American and British objective to find common ground on which internal and external forces both might stand. Washington and London see in that strategy a way to have blacks inherit a going country, not one wracked by black civil war and white flight, and to preclude Cuban-Soviet intervention on the side of the guerrillas. Though results so far have been nil, it remains a worthy strategy. It serves the British-American commitment to a peaceful and stable settlement. Diplomats should continue looking for openings, during the immediate process of working out an interim government and during the preparations for a new constitution and elections, to promote compromise.

But there has to be a limit. To bargain the guerrillas off the battlefield requires that they be offered a certain share of political power. How much? Between the share they think they might get by going political at any particular point, and the share they think they might get by battling on in quest of more, lies the gap between peace and a deepening war.

It is tempting to say that it's not up to the United States to allot shares of power in someone else's country. But that's something of a cop-out. By its involvement, and especially by its refusal to accord at least moral approval to the side - the internal side - going most in the "American" direction of moderate, multiracial democratic rule, the United States accumulates an obligation to choose. We suspect public opinion will be nudging the administration in that direction anyway. The more the internal partners demonstrate their legitimacy, the more they deserve to win American support, and the more likely they will be to get it.