Koijärvi was a municipality for less than half a century. It was carved out of Tammela and Urjala in 1923 — the same year Forssa was separated from Tammela — and it covered 174.5 square kilometres of field, forest and bog. At the end of 1968 it had 2,262 inhabitants. A year later it was gone: 2,103 people went south to Forssa, 159 north to Urjala, and the name survived on lake charts, road signs and letterheads nobody hurried to replace.

It never got around to a coat of arms. Through the 1960s, Koijärvi was one of only two Finnish municipalities without one, sharing that distinction with Uudenkaupungin maalaiskunta. For a place about to hand the country a political movement, it was a modest send-off.

The main village, Kojo, is older than the municipality by centuries. It surfaces in the records in 1600 as Quoiuull, a scribe's attempt at the Finnish adessive — "at Kojo" — which modern orthography writes Kojolla and which Tavastian dialect speakers clip to Kojol. Names here wear down like stones in a ploughed field.

Kojo's church began life as a community hall, built between 1920 and 1922 to plans by the forester Mauno Kallio. A bell tower was added, and on 16 December 1923 Bishop Jaakko Gummerus of Tampere consecrated the building as a church. Its altarpiece, showing Christ in Gethsemane, was painted by Elias Muukka and given to the congregation in 1924; the organ came from the Kangasala works in 1968. The two iron bells travelled furthest of all. Cast in Yaroslavl, they had once hung in a chapel built for Russian soldiers at Tammisaari before ending up calling a Häme farming village to worship.

In 1906 Kojo manor took in an unlikely guest. Maxim Gorky — by then Russia's most famous living writer and a man who had to leave the empire for political reasons — spent three months here, in a house surrounded by rye fields and spruce. Finland was still a Grand Duchy under the Russian crown, close enough to reach easily and separate enough to offer cover. Whether the quiet suited him is not recorded. A plaque marking the stay was unveiled on the manor's main wall in 1968, one year before the municipality that contained it ceased to exist.

In April 1979 landowners around the lake started drainage work without authorisation, the water court case still unresolved. On 21 April, conservationists built a dam to hold the level up. When machines arrived to tear it out, protesters chained themselves to them. Police hauled them off, the dam came down, and the activists built it again — and again. By 1980 the argument had travelled to Helsinki, where hunger strikers took up position in Senate Square.

The reckoning came at the Tammela district court in May 1982. A hundred and twelve people were charged, some of them minors. That December, 101 were convicted of obstruction and causing harm, with fines reaching 2,000 markka. On appeal, the Supreme Court declined to accept civil disobedience as a defence.

They lost in court and won nearly everything else. The lake was never drained. Finland established a national environmental administration in 1983, the same year Ville Komsi entered parliament; Osmo Soininvaara followed in 1987, and Pekka Haavisto became the country's first Green environment minister in 1995.

Koijärvi has been protected since 1992 and sits inside a Natura 2000 site of roughly 242 hectares, which is not quite the same thing as being saved. Shallow lakes silt and grow in; reed and sedge advance a little further each year. Black-headed gulls and coots have disappeared from the breeding population entirely. Cranes have held their ground, swans and marsh harriers still nest along the shore, but the open water that made this the region's finest bird lake keeps shrinking.

Kojo carries on quietly. There is a primary school for grades one to six with a daycare in the same building, and a small grocery. Twenty kilometres down the road, Forssa handles everything else.