ENEstablished after the Polish-Bolshevik war by the Treaty of Riga of 1921 and incorporation of “Central Lithuania” in 1922, the Eastern borders of the Second Republic of Poland (1918-39) divided a territory that had previously always belonged to one state: first the Commonwealth of Poland-Lithuania, and later, after the Partitioning of Poland - to Russia. Apart from the south eastern section where the border of the USSR followed the former border of Austria-Hungary and Russia and ran along the main course of the Zbrucz River, the border was not natural. It intersected areas that were geographically coherent, bisecting various social, economic, and even family communities. The people living on either side were, to all intents and purposes, of identical ethnicity and denominations. On both sides of the Polish-Lithuanian border (not recognised de iure by Lithuania, by the way) there were clusters of people from the other side: Poles in Lithuania, and Lithuanians in Poland. Both sides of the northern section of the Polish-Soviet border were inhabited by Polish, Belarusian, and Jewish people, and of the central and southern sections - by Polish, Ukrainian, and Jewish. Citizens of Poland, Lithuania, and the USSR had families and friends in the neighbouring states. The central section of the southern border of Poland crossed the forested and marshy terrain of Polesie and was poorly populated, with almost completely undeveloped road, rail, power, and telephone infrastructure. These areas were very poor and backward in both economic and civilisational terms. It should also be remembered that the borders in question separated states that remained enemies, and their very demarcation resulted from military hostilities. All these factors contributed to the specific character of these borders, which were penetrated en masse by smugglers, spies, saboteurs, and provocateurs.Protection of that border was a major problem for a Polish state that was only just developing. It was only the organisation of the Border Protection Corps (Korpus Ochrony Pogranicza, KOP) in 1924 that provided relatively efficient protection of the eastern border, while the southern (with Czechoslovakia) and western (with Germany) borders were protected by the Border Guard (SG). The KOP consisted of crack troops organised into brigades and regiments. Its soldiers had police rights and the Corps as such had its own intelligence which was autonomous from the main organ of state intelligence, namely Department II of the General Staff. Moreover, two agencies of Department II, namely Agency No. 1 in Wilno and Agency No. 5 in Lwów, were included in and ordered to report to KOP intelligence. Thus KOP provided comprehensive protection of the eastern border and eastern borderlands, covering administration, policing, customs, and also counterintelligence. The task of the KOP Intelligence was to manage counterintelligence activity in the border zone in Poland and short-range reconnaissance and intelligence in the USSR and Lithuania. The Soviet intelligence active on the territory of Poland was in the hands of two institutions. Short-range intelligence and counterintelligence was managed by the predecessor of the KGB, the GPU (later transformed into the GUGB, OGPU, NKVD, NKGB, and KGB), which came out of the Cheka. Deep intelligence was managed by the military intelligence, Razvedupr, i.e. the Main Intelligence Directorate of the Red Army which in 1942 assumed the name of GRU. All state intelligence in Lithuania was managed by the military intelligence, or Department III of the General Staff, supported by the political and border police forces and reporting to the Ministry of the Interior.The research covered the files of 56 cases of espionage for the USSR and Lithuania from 1926-38 which were investigated by the Regional Court in Wilno and deposited in the Central Lithuanian State Archive in Wilno, Lithuania. Due to their contemporary strategic situation - between the USSR, Latvia, and Lithuania, and close to the border of Germany (Eastern Prussia) - Wilno and the Wilno voivodeship (today Vilnius and the Vilnius region, and for that reason also the Wilno Court Region) had a high army presence. For this reason it was an area of special interest to Soviet and Lithuanian intelligence. Similar espionage cases were also investigated by other contemporary regional courts whose areas of jurisdiction were adjacent to the eastern border. Of these, the Regional Court in Suwałki mostly investigated cases of Lithuanian (and German) espionage, and the remaining regional courts in Wilno, Lublin, and Lwów (today Lviv) - of Soviet espionage. Altogether 108 people were tried in the examined cases, of which nine were recognised as innocent, four were sentenced to capital punishment, one was sent to the Corrective Institution for Minors, and one case was discontinued. In the remaining cases, prison sentences were passed. On the one hand, criminal espionage cases proved to be the focus of interest of the Lithuanian and Soviet intelligence services, ways of recruiting agents, and methods for working with agents and their contacts with the headquarters. On the other - they showed the field and investigative work of Polish counterintelligence, and the proceedings before the court. They also revealed the social (poverty!) and national background of the events. Of the relatively numerous works on the activity of Polish intelligence and counterintelligence between the two world wars published recently, none were based on a study of court files in espionage cases. [...]. [From the publication]