Yes, he composed the first (surviving) opera. But much more importantly, Monteverdi made the leap to opera across the watershed between the old music, stile antico, polyphonic and static, and the new Baroque, stile moderno, glorying in solo voice and expressive harmony. Bridging the two styles – two textures of music – he created, in a long and highly varied life, madrigals, sacred music and opera, combining and developing the old and new. Limelight magazine has a fine narrative of his role in changing music and drama. Our focus is on opera, but Monteverdi’s was far broader, and his music brought him notoriety before opera. Go here for the fascinating story of his life in the royal and papal courts of Italy, and his innovative artistic work. To the nascent genre of opera, he brought the power of a musical imagination unrivalled in his own lifetime, together with formal skills and psychological insights among the most impressive of any composer in history. He created works of extraordinary stature in nearly every significant form of the day. (Read the complete text)

That first (surviving) opera, L'Orfeo, (1607) was radical in combining daring use of polyphony  with more modern display of voice and harmony. Three years later, he produced the even more radical Vespers. Continuing the above quote, A kind of demonstration piece: an example of what can be done setting texts in different styles, particularly the new theatrical style (the foundation of opera) of which Monteverdi was a great pioneer. Instead of hearing the flowing, closely knit counterpoint … of the preceding generation, you hear something that's half opera and half dance. It's a marvellous mélange of styles. He was over 70 when he wrote the two works regarded as the first modern operas - The Return of Ulysses to His Country (1640) and The Coronation of Poppea (1643). Return of Ulysses is a story-telling venture, following Homer. There’s a great summary here. Acts 1-2 are on YouTube, with English subtitles. Both operas focussed on the development of characters, in the case of Poppea, historically real complex characters, seen in realistic situations and the music told their stories and their situations.