After World War I, in 1919, Ilirija resumed its activities. During this period, other football clubs in Ljubljana also began to emerge, including Šparta, Svoboda, Jadran, and Primorje, while Slovan also resumed its operations. Additionally, Olimp was founded in Celje, Amater in Trbovlje, and SSK Maribor in Maribor.

For a long time, the biggest competitors were NK Ilirija and ASK Primorje, which was founded in Ljubljana by Primorska during the Italian occupation of their territory. Due to the great financial problems of both clubs, in 1935, at the initiative of the then mayor of Ljubljana, Jur Adlešič, a merger took place and the new club SK Ljubljana was formed.

As the strongest Slovenian club of its era, SK Ljubljana participated in federal competitions, which prevented it from competing in the Slovenian championship. The club featured the best Slovenian football players, effectively serving as a representation of the Slovenian national team. SK Ljubljana operated successfully until the outbreak of World War II, which once again brought football activities to a standstill.