Les Troyens is so widely accepted as Berlioz’s greatest work, that the progress of the Berlioz Renaissance is punctuated by performances of it in the opera house and in concert, beginning, arguably, with Sir Thomas Beecham’s moderately abridged 1947 BBC broadcast. Now Boston music-lovers may consider the Berlioz Renaissance to be something of a noble fiction, since his music has had its own secure place in the Boston Symphony repertoire for many years, maturing with Charles Munch’s arrival in 1949. During his tenure he and the BSO performed and recorded several of Berlioz’s most important works, and the recordingsare still considered among the best. Later, both Jean Martinon and Seiji Ozawa continued the tradition most capably, and Berlioz has been one of James Levine’s great enthusiasms since early in his career. Expertise in Berlioz seems to be a prerequisite for the job. Yet, this is the first complete performance of Les Troyens by the foremost Berlioz orchestra in America, which in the past has only played brief excerpts, above all the “Royal Hunt and Storm” from Act IV. Hence these concert performances of Parts I and II on following weeks, culminating in a complete performance on Sunday May 4, are in fact landmarks.