Legal observers agreed yesterday that Manhattan District Attorney Cyrus R. Vance Jr. had no choice but to drop sexual assault charges against Dominique Strauss-Kahn that he could not prove beyond a reasonable doubt, although some said Mr. Vance’s office may have acted carelessly in embracing early on the credibility of the witness it would later repudiate.

The comments came as a judge accepted the prosecutor’s motion to drop the case against Mr. Strauss-Kahn, the former managing director of the International Monetary Fund and possible Socialist candidate for president of France, for allegedly attempting to rape Nafissatou Diallo, a 33-year-old hotel housekeeper, on May 14.