Spain planned to attack Britain's new colony in Australia with a 100-vessel armada as part of an operation to "take the fight to the British in the Pacific", documents show.
The papers, found in Spanish Navy archives, disclose that King Carlos IV approved the strategy following a Spanish exploratory expedition to Sydney in 1793, five years after the British settlement was established.
As the European powers struggled for supremacy in the Pacific, the invasion was intended to ensure the colony would not be used to cause "great harm" to Spain's interests in central and southern America and the Philippines.
Documents found by Chris Maxworthy, of the Australian Association for Maritime History, showed the Spanish intended to unleash a cannon that fired heated cannonballs that could set fire to ships or buildings.
"The plan was to attack Sydney from the Spanish colonies in South America with a fleet of 100 medium-sized boats armed with cannons and 'hot shot'," Maxworthy told the Australian Financial Review. "The goal was the complete surrender by the British and their expulsion from the Australian land mass ... The effect [of the hot shot] would be to ... create multiple fires in the wooden buildings of that era in Sydney."
