1918–1920 A new state, and a border across the town's own hinterlandNový štát a hranica cez vlastné zázemie

In 1918 the Hungarian kingdom collapsed and Fiľakovo found itself in Czechoslovakia — a town where most people spoke Hungarian, inside a state built on a Czech and Slovak majority. The change was fought over: in the summer of 1919 the Hungarian Red Army pushed north into Novohrad and there was fighting around Lučenec before its units withdrew. The Treaty of Trianon of 4 June 1920 fixed the border a few kilometres to the south. Salgótarján, the industrial town Fiľakovo had always traded with, was suddenly in another country.Roku 1918 sa Uhorské kráľovstvo rozpadlo a Fiľakovo sa ocitlo v Československu — mesto, kde väčšina ľudí hovorila po maďarsky, v štáte postavenom na českej a slovenskej väčšine. O zmenu sa bojovalo: v lete 1919 postúpila maďarská Červená armáda na sever do Novohradu a v okolí Lučenca sa bojovalo, kým sa jej jednotky nestiahli. Trianonská zmluva zo 4. júna 1920 ustálila hranicu pár kilometrov na juh. Salgótarján, priemyselné mesto, s ktorým Fiľakovo odjakživa obchodovalo, bol zrazu v inej krajine.