THE GUERRILLAS in Nicaragua have launched a "final offensive" to topple President Anastasio Somoza, who also hopes it will be final, though in another sense of the word. The principal reason it has come to this is that the Somoza dynasty provided no reasonable alternative to the people and social groups desiring political change. Last year the United States led a hemispheric attempt to introduce a process of conciliation from the outside. The guerrillas boycotted the process. President Somoza could have made it work but chose not to. Both sides in effect voted for a test of strength. Many Nicararguans have repudiated Mr. Somoza and his authority has not been legitimized in any popular respect, but he is smart and has a strong army and the outcome remains in doubt. It will be a bloody resolution, but in this case there seems no other way.

The Nicaraguan government, presenting itself as the victim of foreign Communist intervention, complained to the Organization of American States that Costa Rica is aiding the guerrillas. So much the international leper is President Somoza that the OAS hooted him down. Nicaragua then exploited a political beachhead in the U.S. Congress to use - or, really, misuse - a House hearing to charge that Panama (and behind it, Cuba and Venezuela) is aiding the guerrillas. For their own reasons - to nullify the Panama Canal treaties - some conservative legislators showed sympathy, but this will not translate easily into useful assistance in Managua. Supporters of the guerrillas charge that Nicaragua's fellow rightist regimes in El Salvador and Guatemala are helping Mr. Somoza. In fact, foreign support does not seem the crucial factor. Each side has its connections, but the battle will be decided within.

The United States is doing just what it should: Nervously watching. This opens it up to accusations from both sides that its inactivity favors the other but that is tolerable. It went to the verge - Somoza supporters would say past the verge - of applying political pressure to get the dictator to relinquish power last year. Failure left it with no further influence to bring to bear. Failure also meant that the Somozas, rather than quit power through a managed process, may leave (if they leave) under conditions of disorder that could provide an opening to elements that might be or might fall under Cuban influence. That troubles the administration but, to its credit, it is taking the risk, counting on containing it, for the sake of giving the opposition a fair crack at doing the dictator in.