The Spanish-Japanese War (Spanish: Español-Japonés Guerra, Japanese: スペイン戦争) also known was fought during the Second World War, between the Kingdom of Spain and the Empire of Japan, mainly for territorial possessions in the Asia-Pacific region. It started with the Japanese invasion of the Philippines in 1941, then a loose Spanish colony with an independent government. Afterwards, King Juan III of Spain officially declared war against the Japanese Empire, as did Prime Minister Manuel Quezon of the Philippines.
Part of the broader Pacific Theater of the war, it was battled on a Philippine Front: involving the Philippine Archipelago and the Micronesian Front, involving the small Micronesian Archipelago. For much of the war, the Spanish forces were heavily disadvantaged against the superior Japanese forces and for the most part, were defeated on tactical levels. The Spanish lacked the proper aerial and naval forces to send supplies to its troops serving overseas, despite having one of its largest military overseas garrisons in the Philippines.
The Japanese easily drove the Spanish forces out of Micronesia by 1943, after vicious bombardment of the garrisons there, where they took over 1530 Spanish troops and 3,424 native Micronesian soldiers as prisoners. As for the Philippines, they held for four years without proper support, and resorted to hit-and-run tactics and scavenger principles - using captured enemy equipment. Still, they suffered one of their biggest military defeats ever since the Napoleonic Invasion, where more than 1700 Spanish soldiers along with Mexican and Cuban volunteers, and 20,000 Filipino soldiers surrendered Bataan and were taken as prisoners. By 1944, the Japanese forces had been weakened due to the American advance, and many withdrew from the Philippines in an attempt to fight the American offensive.
This in-turn, allowed the remaining Spanish and Filipino forces to advance on weakened Japanese positions: in Bataan, in Manila and Corregidor under the leaderships of Spanish general Arturo Valdez y Ariza, and Filipino general Antonio Luna. The Americans landed on Philippine soil on Leyte, to drive the remaining Japanese back to Manila - where they subjugated the Japanese forces in a pincer-style attack, with the Americans attacking from the south and the Spanish and Filipino forces from the north and west. This resulted in the historic meeting between Arturo Valdez, General MacArthur and Antonio Luna.
In the end, over 3,000 Spanish soldiers (mostly military officers and leaders) perished in the war, along with an additional 43,454 native Filipino and Micronesian soldiers and militant fighters alike. It would be the largest military casualty suffered by the Spanish army in the 20th century and the modern era.
The war had long-lasting effects on Spain and the Philippines. Many Filipinos at this point had begun to question the military capabilities of the Spanish Crown, enough to being rejecting them in favor of a sovereign army. Others had an understanding that the Spanish military wasn't prepared for such a war, and fought valiantly for what they had. This had also established political and diplomatic military relations with the Philippines and the United States, via approval by King Juan III of Spain.