I just finished reading Mosquito Empires: Ecology and War in the Greater Caribbean by J.R. McNeill. The book essentially makes the argument that it was nearly impossible for invading forces made up of people from northern climates to capture locations in the New World tropics. The continuous presence of many tropical diseases, especially yellow fever and malaria, made locals immune to these diseases from a young age but were much more likely to kill visitors who had never been exposed to the disease before.

McNeill uses Cartagena as a case study. The city was already teeming with tropical diseases prior to the British invasion. The diseases regularly killed a third or a half of Spanish galleon crews who stopped at the port for trade fairs. As mentioned above, their prevalence also would have exposed locals to the diseases early in their lives; if the young person survived, he or she would not get the disease again. With the outbreak of war, 29,000 British troops and sailors crossed the Atlantic and attacked the city under the command of Vernon, though only a third of that number actually went ashore under the command of Wentworth. Unbeknownst to the attackers, the Spanish had recently invested significant funds into improving the defenses around the city, which featured forts, walls, and batteries defending almost all routes of possible attack. Interestingly, McNeill points out that Spanish defenders never intended to withstand a foreign invasion, only delay an invasion force by six to eight weeks using their fortifications, at which time tropical diseases would take hold and destroy the invasion force.

British troops assaulted the city’s defenses in several places, using strategies suited to sieges in Northern Europe. They set up batteries, blew holes in Spanish defensive positions with artillery and naval bombardment, then assaulted through the breeches. It was a slow, methodical process---the exact opposite of the strategy they should have employed. In most cases, Spanish garrisons held out for a few days or even weeks at a time, slowing the British advance and allowing disease to spread among the invasion force. Additionally, British naval and army officers did not work particularly well together, leading to inefficiency and discord. Despite their disagreements, British forces took most of the defensive approaches to the city and the navy controlled the harbor. Unfortunately, an attack on one of the last Spanish defensive positions failed miserably. Shortly thereafter, heavy tropical rains descended upon the city for weeks, forcing much of the invaders onto ships or into close quarters to stay dry. This caused disease to spread even faster among the foreigners. Quickly, the invasion force melted away: of the 9,000 British troops who went ashore, 8,000 died (mostly from disease). Lord Elibank wrote:

Thus ended the fatiguing part of the Campaign & it certainly was the most disagreeable one that has been known. This People were more sensible of, that it had none of the Comforts of War, with all the Inconveniences of it, & many others not known to those who take the Field in Peopled Countries and temperate climates. The Distructive part of it ought to take its Date from our disembarkation. The sickness amongst us till then, was no more than might be accounted for by our great fatigue, the excessive Heats, bad water, and salt provisions, and without respect officer and soldier shared alike but hardly were we on board again, when we became sensible how impossible it would have been to have kept the Field much longer, the excessive rains and Thunder that are constantly expected here at this time of the year, and can be compared to nothing of the kind felt in Europe, begun to take place, & as they ever do, brought along with them universal Sickness & Death. We lost above a 3rd of our People as well officers as Soldiers, in 3 weeks that we remained in Carthagena Harbour. (quoted from McNeill 163).