San Juan, Puerto Rico
History
The first people on what is now considered the island of Puerto Rico
were known as Taino 
Indians, and were
virtually wiped out by the invasion of Spanish explorers starting with Columbus
in 1493. Colonizers forced the local population into slavery in the gold
mining industry that had drawn Spanish attention to the island in the first
place.
The site for Old San Juan was chosen for it's defensibility and for optimal control of San Juan harbor. The post served mainly administrative and military purposes in the Spanish Empire until trade was encouraged in 1756 by Spain's Barcelona Company. The encouraged demand for goods created an active smuggling network and combined with the rise of the plantation-owning bourgeoisie, an impoverished Spain struggled to maintain control in their former Caribbean colonial capital. Political, economic, and social turmoil caused by Spain's inability to effectively govern her colony led to The Autonomic Charter, granted by the Spanish government in 1897 to allow Puerto Rico proportional representation in Spain's constitutional government and at home.
Shortly after, the Spanish-American War caused
Puerto Rico to be invaded and occupied in 1898 by US forces. The United
States' victory resulted in the cessation of Puerto Rico to the US, and
for two years th
e island was ruled
by American generals. The transition from Spanish to American possession
was accepted less enthusiastically in San Juan than in the rest of the country
due to it's concentration of Spanish merchants and members of the colonial
bureaucracy.
US influence brought improvements in infrastructure and increased manufacturing and industry to the San Juan region. Roads connecting San Juan to Bayamon and the rest of inland Puerto Rico were upgraded and sanitation systems were introduced in both rural and urban communities. The birth rate quickly overtook the death rate with the improvements in health care and sanitation, leading to a huge surge in the population of San Juan and Puerto Rico in general.
The increased employment opportunities coincided with a mass migration to the United States, New York City in particular. More than 40,000 Puerto Ricans migrated to the mainland in the 1920s alone, drawn by higher wages and unrestricted by immigration regulations that had previously stopped migration.
Most of the jobs created by US occupation were centered in San Juan, which drew much of the remaining rural population to the capital and surrounding industrial areas of Bayamon and Rio Piedras.
The Depression of the 1930s slowed migtration, and between 1930 and 1934, there was a return migration of almost 10,000 people (20% of the mainland Puerto Rican population). Because the capital was much more modern than other cities or towns on the island, the majority of the return migrants settled in San Juan.
The end of World War I signaled the beginning of the greatest exodus that Puerto Rico would see. The abundance of employment opportunities, as well as the expansion of air travel and the reduction in the cost of the trip were all factors that led to a net out-migration of 550,000 Puerto Ricans to the US in the 1940s and 50s.