What was the Onin War?
The Onin War was a ten-year conflict fought mainly inside Kyoto between two coalitions of great families. It settled nothing, killed the men who started it, burned most of the capital, and left the shogunate with a title but no power.
It is the conventional starting point of the Sengoku period. Not because it was decisive — it was the opposite — but because it demonstrated that the central government could no longer control the provinces, and every provincial lord acted on that knowledge afterwards.
Why did it happen?
The Ashikaga shogunate governed through a small number of great families who held multiple provinces as military governors. By the 1460s, three of those families were split by their own succession disputes, and the shogunate had no mechanism to settle them.
The trigger was the shogun's own succession. Ashikaga Yoshimasa, having no son, had persuaded his brother to leave the priesthood and become his heir. A son was then born. The two claims were backed by the two most powerful men in the capital, Hosokawa Katsumoto and Yamana Sozen, who were already opposed on several other disputes.
By 1467 both had brought provincial armies into Kyoto. Fighting broke out in the streets around their compounds, and the districts between them burned.
Forces and sides
Chronicles give very large figures for both coalitions, in the range of one to two hundred thousand men in total. These numbers come from literary accounts written to convey scale and should not be treated as counts.
What is clear from the record is that both sides brought provincial levies into a city that had no way to feed or police them, and that much of the destruction was done by irregulars fighting for plunder rather than by either commander's plan.
What happened?
The first year was the worst for the city. Fighting moved street by street through the northern districts, and fire destroyed temples, palaces and the archives held in them. By 1468 the two armies had dug in behind ditches and towers across a ruined capital.
After that the war stagnated. Both principals died in 1473 — within two months of each other — and the fighting continued anyway, because the provincial contingents had their own reasons to stay. The shogun Yoshimasa retired to build a villa, now the Silver Pavilion, and devoted himself to garden design and the tea aesthetic while the city burned.
The armies finally dispersed in 1477. Nobody had won. The provincial lords went home to find that their deputies had taken over in their absence.
Turning point
The decisive change was not on any battlefield. It was that the war taught every ambitious man in Japan that authority now belonged to whoever held land and soldiers, not to whoever held an appointment from Kyoto.
This is the pattern the era named gekokujo — "the lower overthrows the higher". Deputies displaced governors, retainers displaced deputies, and within a generation most of the old order had been replaced by men who simply held their territory by force.
Outcome
Kyoto lay in ruins and would take a century to recover. The Ashikaga shogunate survived on paper until 1573, but from 1477 onward it controlled little beyond the capital and was frequently expelled even from that.
Power devolved to the provinces, where a new kind of lord — the daimyo — governed his own territory, taxed it directly, and fought his neighbours without reference to any central authority.
Why it mattered
If you want one event to explain why Japan spent the next century at war, this is it. The Onin War did not create a new order; it removed the old one and left nothing in its place.
Everything that follows on this site — Oda Nobunaga's rise, the wars between Takeda Shingen and Uesugi Kenshin, the eventual unification — happens in the vacuum the Onin War created.