In the course of the First World War, the Russian, German, Austro-Hungarian and Ottoman Empires ceased to exist. On their territories new independent states were formed.
Among them was Poland, which proclaimed its independence in November, 1918. Earlier, the Soviet Russia unilaterally annulled all documents adoptedby the Russian Empire since the partitions of Poland. In the situation of unsettled border issue with Russia the Polish government, instead of searching for peaceful solution, resorted to the policy of territorial claims and occupation of the Ukraine, Belorussia and Lithuania. During the Polish-Soviet War the Brest Fortress and the city repeatedly passed from one country to another: on February 9, 1919, the Polish troops occupied Brest-Litovsk, advancing towards Minsk. On August 1, 1920, the Red Army seized Brest-Litovsk and its fortress. The Soviet power lasted in the city for 18 days. On August 19, 1920, the Polish troops recaptured Brest.
On March 18, 1921, the Riga Peace Treaty was signed between Poland, the Soviet Russia (acting also on behalf of Soviet Belorussia) and Soviet Ukraine. The treaty ended the Polish-Soviet War and establishedthe Polish-Soviet borders. However, it divided Belorussia into 2 parts. Western Belorussia wasceded to Poland and the Eastern Belorussia remained a Soviet republic, that later joined the Soviet Union as the Belorussian S.S.R. Brest-Litovsk (1923 –1939 Brest-on-Bug) became the administrative centre of Poleskie Province of Poland. The Polish Army units were stationed in the Brest Fortress. Officers’ families were also accommodated there.
The Polish Army was retreating to the east, suffering heavy losses.
There were first casualties among the military in the fortress and the population of the city during the German air raid on Brest on September 2, 1939.
On September 14, 1939, the German 19th Army Corps under command of Lieutenant-General Heinz Guderian occupied Brest-on-Bug, but the assault of the fortress failed.
On September 14-17, 1939, the surrounded Polish garrison (2,500 people strong) was defendingthe Brest Fortress against outnumbering Wehrmacht troops, suffering about 40% loss of its men, being killed or wounded.
On the morning of September 17, the fortress was seized by the Wehrmacht units.
On the same day the Red Army crossed the western state border near Minsk, Slutsk, and Polotsk to avoid German occupation of Western Belorussia.
On September 22, 1939, the units of the Red Army under command of S.M. Krivoshein entered Brest and the Wehrmacht units left the city, crossing the Western Bug River. Since the autumn of 1939, the river became the border between the USSR and Germany.