In 1625 Puerto Rico was covetously eyed by Holland, whose traders and merchants desperately wanted a foothold in the West Indies. Spearheaded by the Dutch West India Company, which had received trading concessions from the Dutch Crown covering most of the West Indies, the Dutch armies besieged El Morro Fortress in San Juan in one of the bloodiest assaults the fortress ever sustained. When the commanding officer of El Morro refused to surrender, the Dutch burned San Juan to the ground, including all church and civil archives and the bishop's library, by then the most famous and complete collection of books in America. Fueled by rage, the Spanish rallied and soon defeated the Dutch.
In response to the destruction of the strongest link in the chain of Spanish defenses, Spain threw itself wholeheartedly into improving and reinforcing the defenses around San Juan. King Philip IV justified his expenditures by declaring Puerto Rico the "front and vanguard of the Western Indies and, consequently, the most important of them and most coveted by the enemies of Spain."
Within 150 years, after extravagant expenditures of time and money, San Juan's walls were considered almost impregnable. Military sophistication was added during the 1760s, when two Irishmen, Tomas O'Daly and Alejandro O'Reilly, surrounded the city with some of Europe's most up-to-date defenses. Despite the thick walls, however, the island's defenses remained precarious because of the frequent tropical epidemics that devastated the ranks of the soldiers; the chronically late pay, which weakened the soldiers' morale; and the belated and often wrong-minded priorities of the Spanish monarchy.
