On March 9, 1842, Giuseppe Verdi's Nabucco premiered in Milan, Italy. One of the highlights of the opera is the Hebrew Slaves' chorus, "Va pensiero". It is traditionally encored, as in this performance. This BBC website provides some background to explain its political significance:
So when Italians sang the Chorus of the Hebrew Slaves at Verdi’s funeral procession, it wasn’t just because it was a catchy tune they knew the words to. Its subject – the Israelites giving poignant voice to their longing for the promised land – had become a powerful analogue for the long-frustrated desires of the Italian people. When they cried “Viva Verdi!” during the funeral procession, they were still acutely conscious of the slogan’s double meaning and its clandestine resonance for the agitators of ‘the Risorgimento’, as the cause of Italian nationalism was known. The letters VERDI also spelled out the name of the King of Sardinia who, in 1861, finally took the throne of a unified nation for the first time since the 6th Century – Victor Emmanuele Re D’Italia.
But this article from The Guardian argues that the political impact of the chorus is more complicated than that:
The most remarkable negative evidence comes from revolutionary Milan, in which so much of Verdi's early activity was centred. In March 1848, for a few brief months, the Milanese drove the Austrians out of their city. Artistic events in the wake of this insurrection were surprisingly plentiful: several theatres started up with patriotic plays; operas appeared soon after; Verdi's publisher Ricordi advertised a long series of patriotic hymns, including one by Rossini. Milan experienced a period of heady freedom; artistic endeavours were for the most part geared unequivocally to the political situation. But during the entire time, there was virtually no mention of Verdi. The composer supposed to have inspired the masses to the barricades, the very artistic symbol of the Risorgimento, was somehow ignored in the press of events.
By August, the Austrians were back in Milan. La Scala re-opened at the end of the year. In the midst of an extreme clampdown on any behaviour that could lead to further civil unrest, the authorities staged revivals of three of Verdi's most popular operas; two others (one of them Nabucco!) followed soon after. It seems impossible that any of these works had been actively associated with the failed revolution or its build-up.
Alas, the choice comes again: the truth or the legend? "When the legend becomes fact, print the legend." John Ford's movies (The Man Who Shot Liberty Valance and Fort Apache) chose the latter, but let us entertain the former. Should opera companies stop encoring the chorus because the opening night encore is a deceptive legend?
The lyrics of "Va pensiero" are based on Psalm 136, which Phillipe De Monte and William Byrd set to music during the Recusant/Penal era in England, as this CD, The Word Unspoken explores:
The texts reveal the Catholic community’s sense of isolation (How shall we sing the Lord’s song in a strange land?—Quomodo Cantabimus) and bereavement (Jerusalem is wasted—Ne Irascaris), and the elaborate, poetic nature of the encoded messages distributed within it through music.