On 17 September 1939 Soviets broke international law

On 17 September 1939, around three o’clock in the morning, the First Deputy People's Commissar for Foreign Affairs of the USSR, Vladimir Potemkin, had handed a diplomatic note to the Polish ambassador in Moscow, Waclaw Grzybowski, in which he cited the alleged collapse of the Polish state, the flight of the government and concern for the "brotherly Ukrainian and Belorussian population" living on the territory of the Second Polish Republic as the reasons for the Soviet invasion of Poland and the annulment of the Polish-Soviet non-aggression agreement. The note was not accepted by the Polish ambassador.

An hour later, the Soviet armed forces attacked Poland, thus implementing the provisions of the Molotov-Ribbentrop Pact of 23 August 1939, which provided for the joint aggression of Germany and the USSR against the Republic of Poland, the seizure and partition of its territory and the de facto liquidation of the Polish state.
The Russian agency Ria Novosti repeated Soviet propaganda in its article devoted to the Soviet aggression on the Second Polish Republic in 1939. It read, among other things, that "in the early morning of 17 September, a memo was handed over to the Polish ambassador in Moscow stating that 'the Soviet government had given an order to the supreme command of the Red Army to cross the border and to take into custody the lives and property of the population of western Ukraine and western Belarus.' Subsequently, Vyacheslav Molotov, Prime Minister of the USSR, gave a radio address in which he pointed out that 'the events triggered by the Polish-German war showed the internal failure and obvious incapacity of the Polish state, that the Polish governing environment had gone bankrupt in the shortest possible time, that nobody knows where the Polish government is. The Polish state and its government had effectively ceased to exist and, in view of this situation, the treaties concluded between the USSR and Poland were no longer valid," Molotov stressed.
Thus, the USSR recognised Poland as a non-existent state and unilaterally broke all agreements previously concluded with the Polish government, declaring them null and void.
When the Red Army crossed the border, the Polish state had been engaged in an unequal struggle with the German aggressor for more than two weeks, but it did exist as the state, and the legal government of the Second Republic did not leave the country until the night of 17-18 September, after receiving news of the Soviet aggression.

17.09.23

The agreements Soviets broke included the 1921 Treaty of Riga, which ended the Polish-Soviet War of 1919-1920 and established the course of the eastern border of the Second Polish Republic, and the provisions of the 1932 non-aggression pact between Poland and the USSR. The latter forbade either side to attack the other either alone or in agreement with a third party.
The USSR broke not only the provisions of the Polish-Soviet non-aggression treaty, but also the ‘Convention for the definition of aggression’, signed by Poland and the USSR and a number of other countries in London in 1933. Article II of the Convention stated that a state is considered an aggressor if it has made an attack with its armed forces on the territory of another state, even without a declaration of war. The Red Army entrance on Polish territory exhausted the definition of aggression contained in the aforementioned convention.
Despite the initial disorientation of the Polish authorities due to the lack of a declaration of war, the Polish government did not have the slightest difficulty in assessing the actions of the USSR. In his address to the nation on 17 September, Poland’s President Ignacy Mościcki unequivocally described Soviet military actions as an act of aggression. It was clear that the Soviets had stabbed the Republic of Poland in the back, which ultimately determined the fate of the September Campaign.

The USSR directed around 1.5 million soldiers, over 6,000 tanks and around 1,800 aircraft against Poland. Despite the order of Commander-in-Chief Edward Rydz-Śmigły not to fight the Bolsheviks, the soldiers of the Border Protection Corps and improvised Polish forces in Vilnius and Grodno were resisting the aggressor. About 2,500 Polish soldiers died in the fights with the Red Army, and about 20,000 were wounded or missing in action. About 200,000 Poles, including 10,000 officers, were taken prisoner by the Soviets.
Another article published by a pro-Kremlin website ukraina.ru stated that Soviets stopped at the Curzon Line: "The USSR sent troops into the territory of Western Ukraine (Volhynia and Galicia) and Western Belarus on the so-called 'Curzon Line', along which the West repeatedly proposed to demarcate the border between Poland and Soviet Russia. These lands, inhabited mainly by the Ukrainian and Belarusian populations, were officially incorporated into the Ukrainian and Belarusian SSRs respectively."

This is not true. The Curzon Line was the demarcation line proposed by British Foreign Secretary Lord George Curzon in his note of 11 July 1920 to the Soviet People's Commissar for Foreign Affairs, Georgy Chicherin. It was similar to the border between the Polish-Lithuanian Commonwealth and Russia after the Third Partition of Poland (of 1795) and ran along the Bug River. From the Lithuanian border it ran along the Neman River, further west from Grodno, in a manner similar to the present Polish-Belarusian border (leaving, however, the Bialowieza Forest on the Soviet side, while the Augustow Canal as well as the town of Sapotskin were on the Polish side), down to the Bug River in the town of Niemirow, and then along the Bug River to the former border of the Russian and Austrian partitions near Krylow, where it ended, leaving the division of Galicia unspecified.
Contrary to what the author of the fact-checked publication claimed, this was not a draft of the Polish-Soviet border proposed by the Western states, but only a proposal for an armistice line between the Soviet and Polish armies fighting in the 1920 war. After the end of the war, the state border was finally defined in the Riga Treaty of 18 March 1921 and was shifted in relation to the Curzon line by about 200 km in an eastern direction, thus in favour of Poland.

Soviets carried out the attack on two fronts: the Belarusian and Ukrainian and did not stop at the Curzon line: after taking Vilnius and Grodno on 22 September, it occupied Suwalki, and on the Ukrainian front—after capturing Chelm and Zamosc, on 28 September—it entered Lublin (these cities, in Lord Curzon's conception, were to remain Polish).
Stalin abandoned the original secret amendment on reaching as far as the Vistula line in order to add credibility to the argument about the Red Army's alleged defence of national minorities: Ukrainians and Belarusians. This was the Soviet’s pretext for entering Poland.
Besides, before 17 September, part of the area east of the Vistula River had already been occupied by the Wehrmacht. However, the Germans withdrew to the demarcation line, agreed with the Soviets on 23 August 1939. Under the terms of the Molotov-Ribbentrop Pact, Germany ceded to the USSR, among other things, Bialystok (which, according to Curzon's conception, also should have remained Polish).

As early as 22 September 1939, Soviet and German forces held a joint victory parade in Brest-on-the-Bug. On 28 September, an additional treaty on borders and friendship was signed, which shifted slightly the demarcation line between the Polish lands occupied by the Third Reich and the USSR. As a result, the border was established on the line of the San, Pisa and Narew rivers.
As a result of the Soviet aggression against Poland in September 1939, the USSR annexed territories with a total area of approximately 200,000 square kilometres inhabited by 13.2 million people. The annexed territories were then incorporated into the Lithuanian, Belarusian and Ukrainian Soviet Republics.
Further consequences were the mass repression of Poles living in the annexed territories and the plundering of Polish national property and private assets taken deep into the USSR. Over 200,000 Poles—officers, policemen, landowners and lawyers—were arrested. Citizens of the Second Polish Republic were forced to accept Soviet citizenship. Around 1,350,000 Poles were deported to Siberia; and about 22,500 officers and policemen were killed in Katyn, Kharkov and Tver.