The dispute was about trade, and trade meant power. Juan Manuel de Rosas, who governed Buenos Aires and ran the foreign affairs of the Argentine Confederation, insisted that commerce with the inland provinces pass through his custom house. Britain and France wanted to sail straight upriver to Santa Fe, Entre Rios, and Corrientes - and to Paraguay beyond - without asking his permission. When Rosas declared the Confederation's rivers closed to foreign warships, the two European powers simply did not recognize the claim. In 1845 they assembled a joint fleet and pointed it up the Parana. The question was no longer diplomatic. It was whether a young South American nation could control the water running through its own land.

The Spanish word vuelta means a turn, and the Parana takes a hard one here. The river narrows to roughly 700 meters, and a cliff rises sharply above the right bank - high ground a defender could not ignore. General Lucio Norberto Mansilla, Rosas's brother-in-law, read the geography like a tactician. He strung three thick chains across the channel, suspended from twenty-four requisitioned barges, the work overseen by an Italian immigrant named Filipo Aliberti, whose own boat would sink in the fighting. Along the bluff Mansilla planted four batteries - thirty cannon, many of them bronze pieces throwing eight to twenty pounds of iron - served by 160 gauchos. Two thousand men waited in the trenches. The river itself had become a fortress.

The allied squadron carried the most advanced machinery of the age: partially armored steamers, rapid-fire guns, Congreve rockets that arced down trailing smoke. For hours the Argentine batteries and the European broadsides hammered each other across the narrow water. The chains held longer than anyone expected, but sailors eventually hacked them apart under fire, and the fleet ground through. The cost was brutal and unequal. Roughly 150 Argentines were killed. Mansilla himself, leading a bayonet charge along the bank, took shrapnel in the chest and had to hand command to Colonel Francisco Crespo. The Europeans lost far fewer men and won the passage. By every tactical measure, it was a defeat for Argentina.

Yet the victory curdled. The fleet that forced its way upstream was attacked again and again on the way back - at Tonelero, at Angostura del Quebracho - and merchant ships were lost. Of ninety-two merchantmen waiting downstream, only about fifty bothered to continue. The arithmetic was plain: the Parana could be forced, but never cheaply, and never for long. Within four years Britain signed the Arana-Southern Treaty (March 1849) and withdrew its fleet; France followed with a treaty of its own (October 1850). Both powers acknowledged Argentine sovereignty over the inland rivers, subject only to Argentine law. The exiled liberator Jose de San Martin, hearing of the stand, wrote to his friend Tomas Guido that the invaders had learned Argentines were not empanadas to be swallowed whole. There is a small, telling coda: until 1948 the Paris Metro had a station named Obligado, renamed Argentine as a gesture of goodwill in the year after Eva Peron visited France. In 1974 Argentina made 20 November its Day of National Sovereignty, and in 2010 a national holiday.