In 1739, the riches that Spain derived from the New World were well known throughout Europe. Huge quantities of silver were shipped from Peru, carried over the isthmus at Panama and then loaded on another ship at Portobelo bound for Spain. Other ships carried luxury goods from Manila to Acapulco from where they were taken to Vera Cruz and loaded along with Mexican silver. Spain's Caribbean possessions provided sugar, tobacco, dyes and spices.
Britain had negotiated a treaty (the Asiento) that allowed the South Sea Company to send one trading vessel per year to Spanish territory plus supply slaves but private British vessels, many operating out of Jamaica, carried illegal cargoes which the Spanish attempted to intercept. After numerous incidents and with old rivalry, tensions increased leading to the War of Jenkins' Ear.
Various schemes were proposed to attack Spanish possessions. Edward Vernon captured Portobelo in November 1739 with just six ships, and a second squadron to be led by George Anson, was to sail around Cape Horn with six warships carrying 500 troops with instructions that might be described as ambitious. These were to capture Callao in Peru (the port that served the nearby capital Lima) and if possible take Lima as well, capture Panama with its treasure, seize the galleon from Acapulco and lead a revolt of the Peruvians to Spanish colonial power. An earlier proposal to also capture Manila was dropped.
It appears that a conflict of interest was, at least in part, behind the unreasonable orders given to Anson by the Duke of Newcastle on 28 June 1740. The attacks were suggested by Hubert Tassell and Henry Hutchison, previously factors (agents) of the South Sea Company who had a significant recent information about the area which the government lacked but who also stood to gain if the area was opened up to British trade. Supporting this view was that the squadron was to carry £15,000 of trade goods. Given the length of the voyage, it was expected that it would be necessary to buy provisions along the way. In friendly ports, bills of exchange would have been used. In enemy ports it may not have been possible to buy anything, though outright seizure would be possible if the town was captured. The relationship of Tassell and Hutchison to the squadron was further complicated by them supplying victuals (food) and that they would come along on one of the ships to oversee the trade goods.
The squadron based in Portsmouth was composed of six warships:
- Centurion (a fourth-rate ship of 1,005 tons, 60 guns, 400 men and the see model)
- Gloucester (853 tons, 50 guns, 300 men)
- Severn (853 tons, 50 guns, 300 men)
- Pearl (600 tons, 40 guns, 250 men)
- Wager (599 tons, 24 guns, 120 men)
- Tryal (200 tons, 8 guns, 70 men)
Two merchant vessels, Anna and Industry, would carry additional supplies.
The provision of 500 troops was farcical. No regular troops were made available so 500 invalids were to be collected from the Chelsea Hospital. In this case, the term invalid referred to soldiers that were too sick, wounded or old for active duty but might be able to perform lighter duties. In any case, on hearing details of the proposed voyage, those that could get away did and only 259 came aboard, many on stretchers. To make up for the 241 missing, marines were ordered aboard but these were such fresh recruits that few of them had yet been trained how to fire a gun.
The squadron was as ready as it was going to get by mid-August but strong winds kept the ships in harbour. Before heading to South America, Anson was required to escort a huge fleet of transports and merchant vessels out of the English Channel and the initial attempt to get to sea was abandoned as ships crashed into each other. Finally the squadron sailed from Spithead on 18 September 1740 overseeing a convoy of 152 ships.
Unfortunately, with the long delays, French agents had picked up word of the expedition and passed the information to Spain. In response, they sent five warships under Admiral Pizarro to lie in wait near Portuguese Madeira which was neutral territory and to be Anson's first port of call.