The Dutch East India Company had controlled southern Formosa from Fort Zeelandia since the 1620s, while Spain maintained a foothold in the north through its garrison at San Salvador, a fortified position on a small island in the Bay of Jilong - modern Keelung. The arrangement was unstable but endurable, each European power occupying its corner of an island neither fully understood. What tilted the balance was gold. Chinese merchants traveling between the two colonial zones brought word to the Dutch that the Spanish were considering abandoning Formosa altogether, waiting only for royal permission from Madrid. The Dutch had heard persistent reports of gold mines in the northeast mountains, and they concluded that prospecting was impossible as long as Spanish soldiers controlled the northern harbors. More practically, they had made contact with the indigenous people of Danshui - present-day Tamsui - and found willing allies. The combination of opportunity and alliance tipped the decision: the Dutch would attack.

In August 1641, a Dutch expedition sailed into the Bay of Jilong. The Spanish, warned by an indigenous informant, prepared for assault. Dutch soldiers landed on the shore across from San Salvador island, and roughly five hundred aboriginal allies from the north accompanied them. They entered the village of Kimaurri without resistance - the Spanish governor had refused to shelter the local population inside the fortress, and most had fled to the mountains. The Dutch spent the night in the abandoned village. The next morning, they climbed the hill behind Kimaurri and studied the Spanish defenses by telescope, methodically counting infantry and assessing fortifications. Despite outnumbering the Spanish and commanding hundreds of indigenous fighters, the Dutch commander reached a pragmatic conclusion: he lacked sufficient artillery to break the fortress walls. A proper siege required cannons he did not have. The Dutch withdrew, burning Kimaurri as they departed - a calculated act of destruction that served notice of what they could do even when they chose not to finish the job.

The Spanish watched the Dutch leave from the walls of San Salvador. What struck them was not the Dutch soldiers but the indigenous force arrayed behind them. One Spanish observer wrote that the Dutch had assembled fighters from the entire Danshui River valley and all the villages under their jurisdiction. Watching from the fortress, the Spanish saw the indigenous warriors deployed at intervals across the hills and beaches and realized they were facing an army. The word the Spaniard chose was telling - these were not scattered allies or hired auxiliaries. They were an organized military presence, and their alignment with the Dutch represented a fundamental shift in the balance of power on Formosa.

On their return voyage south, the Dutch stopped in Danshui and formalized an alliance with the indigenous communities there. Emissaries from Danshui subsequently traveled to Fort Zeelandia, the Dutch headquarters in the south, and according to Dutch records, officially transferred sovereignty of their lands - following the same pattern that southwestern plains villages had established with the Dutch during the 1630s. The diplomatic consequences were devastating for Spain. The Danshui region, once central to Spanish influence in northern Formosa, became hostile territory overnight. The Spanish governor wrote to Governor-General Corcuera in Manila that the indigenous peoples had turned against them entirely. He could no longer persuade them to cooperate in even minor matters. His complaint carried a bitter edge of recognition: the aboriginal communities allied with whoever demonstrated the greater strength. The burning of Kimaurri beneath the fortress walls had been humiliation enough. The Dutch had not needed to capture San Salvador to defeat the Spanish. They had simply proven, in full view of every indigenous community in the region, that Spain could not protect its allies. The Second Battle of San Salvador, one year later, would finish what the first had started.