Early on, Sidzikauskas became involved in various proceedings of the League of Nations. As a representative of Lithuania, he signed such international agreements as the Barcelona Convention and Statute on Freedom of Transit of 1921 and the Slavery Convention of 1926. In April 1921, Sidzikauskas became a member of the Lithuanian delegation, led by Ernestas Galvanauskas, to negotiate with the Second Polish Republic regarding Vilnius Region which Poland captured as a result of the Żeligowski's Mutiny in October 1920. The negotiation was mediated by Paul Hymans whose plan was to transfer Vilnius to Lithuania if it agreed to join a federation with Poland. Lithuanian public vehemently opposed such plans and Galvanauskas even suffered an assassination attempt. After the Klaipėda Revolt in January 1923, Sidzikauskas became one of the negotiators of the Klaipėda Convention which was concluded in May 1924.