As she often does when she's introducing new music to the Baltimore Symphony Orchestra's audience, Marin Alsop turned around on the podium Saturday night and talked a bit about John Adams' Doctor Atomic Symphony. In her black-satin suit with the magenta cuffs, she explained how the piece was based on the invention of the atomic bomb in New Mexico in 1945 and that the weapon's capacity for "utter annihilation" would mostly be represented "by the timpani, though the brass will help out." She later added the symphony "does what all art should do: It reminds us of our history and what we shouldn't do." That's a curious definition of art, and it understates the achievement of Adams' extraordinary work to describe it as a didactic warning against nuclear weapons. The symphony is at its best when it evokes the struggle between what we could do and what we should do. We could cut down all the trees in our backyard, but should we? We could design a financial derivative that will make us rich at the expense of many others, but should we? We could design a weapon of mass destruction, but should we? Physicist Robert Oppenheimer faced the latter question in 1945, and Adams captures his dilemma by respecting both halves of his choice. Yes, such a weapon's destructiveness would be horrifying, but the secrets of the atom were just sitting there, a wonderful puzzle waiting to be solved—and why shouldn't he do it before someone else did? Doctor Atomic was originally an opera, with Peter Sellars adding words from scientific reports and John Donne's poetry to Adams' music. The composer later distilled that music to a 25-minute symphony, and it works better without the footnote-like words. The symphony is a series of contrasts—between turbulent passages that suggest a vision of what the weapon might do and quieter passages that suggest a late-night struggle of conscience. Pushing and pulling at that conscience are the barking voice of General Leslie Groves, commanding Oppenheimer to move forward, and the keening voice of the American-Indian maid Pasquelita, begging the scientist to hold back. The symphony works only because each side of the argument is equally persuasive. Alsop was much better at conducting the work than explaining it. The turbulent sections pitted the strings, seeming to buzz with static electricity, against the thundering brass and percussion. The decision sections were just as tense, though much quieter, thanks to the nicely understated use of unresolved chords and counterpoint. Even without text, the roles of Groves, Oppenheimer, and Pasquelita were brought to life with sharp definition by trombonist John Vance, trumpeter Andrew Balio, and horn player Philip Munds respectively. For all its American themes and modern touches (the prominent percussion section and the dissonant accents), the Doctor Atomic symphony still uses the instrumentation and structures of European art music. A similar cross-cultural collision took place in 1892 when Czech composer Antonin Dvorak came to the United States for an extended stay and decided to compose a symphony inspired on the nation's industrial revolution and African-American folk songs. The resulting work, From a New World, has been popular since it premiered in 1893, but its motifs are as often East European as North American. Again Alsop allowed both sides of a conversation to have their say. She had the woodwinds clearly state the opening phrase from the old spiritual, "Swing Low, Sweet Chariot," before they veered off on a tangent of European inspiration. She coaxed a gorgeous English horn solo from Jane Marvine in the second, slow movement. But Alsop also gave the spirited dance melody in the third movement its essential Bohemian identity. And she pulled all these elements together in a rousing, slam-bang finish that represented not only America's gung-ho spirit in the 1893 but also the BSO's muscular cohesion in 2010. Between the two symphonies was Mendelssohn's violin concerto, about as European a piece of music as one can imagine. Unlike many concertos, this one is mostly a soloist's vehicle, with the orchestra relegated to a supporting role. Fortunately, Alsop had invited a terrific violinist, 25-year-old Stefan Jackiw, as the soloist. He handled the concerto's fast and showy parts with aplomb, projecting confidently and never straining. But he was most impressive on the slower passages, where he gave the impression of singing through his instrument. Dressed all in black with his knees always bent a bit, Jackiw would round off juicy melodic phrases and allow them to tail off into an intimate stage whisper, almost as if sighing with regret.