In 1817, most of Venezuela remained under Royalist control. The independence movement survived in pockets, sustained by guerrilla leaders operating in the sparsely populated interior. Among them was Manuel Piar, a mixed-race general whose forces operated in the Guayana region along the lower Orinoco. Piar understood what Bolivar also understood: whoever held Angostura held the river, and whoever held the river could move troops, supplies, and information across the vast interior. In January 1817, Piar had already tried to take the city by force and failed. He left Manuel Cedeno behind with enough troops to keep it under siege while he turned his attention to cutting the Royalists' supply lines -- a strategy that would prove decisive.

The siege created catastrophic conditions inside Angostura. Hunger ravaged soldiers and civilians alike, pushing both to desperation. On 8 March, the Spanish dispatched a relief convoy of 35 ships from San Fernando de Apure, carrying between 1,000 and 1,500 soldiers under Brigadier Miguel de la Torre. The convoy arrived on 27 March, but De la Torre's attempt to reopen supply lines by conquering Caroni ended in disaster at the Battle of San Felix on 11 April, where Piar's cavalry routed the Royalist forces in just half an hour. With the supply route severed for good, the clock began ticking on Angostura's defenders. A night assault by Piar on 25 April failed after four hours of fighting, costing the Patriots 7 officers and 78 soldiers, but it hardly mattered -- the city was dying from the inside.

On 4 May, Simon Bolivar himself arrived at Angostura with his fleet, having landed in Guayana in March. His first act was political rather than military: he relieved Piar of command, sending him to Upata, and placed Jose Francisco Bermudez in charge of the siege forces. The move was calculated. Piar had done the hard work of the campaign, but Bolivar needed undivided command, and Piar -- popular, capable, and independently minded -- represented a threat to that unity. The siege ground on through May, June, and into July, the Royalist garrison shrinking as disease and hunger took their toll. Of the soldiers inside the walls, only 300 remained fit for duty. The rest filled the hospitals.

On 17 July, General La Torre finally accepted what had become inevitable. He ordered the entire population -- 1,800 civilians and 2,000 soldiers and sailors -- to board every brig, schooner, and barge in the port and flee downriver toward the Orinoco delta. They carried their belongings, valuables, and archives, abandoning a city that had been Spanish for centuries. Luis Brion's Patriot ships, under Bolivar's orders, harassed the convoy relentlessly. Panic spread among the fugitive vessels as they entered the labyrinth of channels in the delta. La Torre and his officers escaped to Grenada aboard the warships, but most of the other vessels were captured or vanished into the maze of waterways. It is estimated that more than half of the evacuees and their possessions ended up at the bottom of the river -- a human catastrophe that the military histories tend to compress into a single sentence.

Bermudez's troops occupied Angostura while the last evacuees were still departing. When Bolivar entered the city on 19 July, he found only a handful of starving people who had been left behind. The victory gave the Patriots a base of operations in Guayana and control of the Orinoco basin, allowing them to link up with Jose Antonio Paez and his llanero horsemen, and eventually to push into the interior of New Granada. Bolivar declared Angostura the temporary capital of his Third Republic of Venezuela -- though Caracas would not fall until June 1821. The triumph also came at a bitter cost: in October 1817, Bolivar had Manuel Piar, the general most responsible for the Guayana campaign's success, arrested, tried, and executed. The Liberator's power was thereafter absolute. From this battered city on the Orinoco, Bolivar would organize the campaigns that ultimately freed five nations.