"From historical point of view, these lands were primordially Slavic":
international law aspects of formation of the Kaliningrad region
The territory of East Prussia first became a part of Russia in the middle of the 18th century. As a result of the Seven Years' War, the Prussian lands were annexed to the Russian Empire. And only the death of Empress Elizabeth of Russia and the accession to the throne of Peter III prevented Russia from becoming firmly established in the Southeastern Baltic as early as in the middle of the 18th century.
In 1941 Germany attacked the Soviet Union and unleashed the war, which became the Great Patriotic War for Soviet people. The formation of the Kaliningrad region is directly connected with the results of that war. It should be noted that even before the outbreak of the war, East Prussia was perceived by the architects of Soviet foreign policy as a potential threat to the territorial integrity of the USSR. The incorporation of the Baltic states into the Soviet Union in 1940 occurred, among other things, in order to move the Soviet border to the West away from Moscow and Leningrad, as well as prepare new frontiers for repelling possible German aggression. By the way, Soviet military leaders were not alone in their views. A similar position was adhered to by Polish politicians, who believed that East Prussia was a springboard for an attack on Poland and its eastern neighbours.
Poland was the first among other states to declare its rights to East Prussia. After the defeat of the Polish army in September 1939, Polish politicians in emigration, unexpectedly, despite the unfavorable state of affairs at the front for their troops, put forward the thesis that it was necessary to include East Prussia into post-war Poland, because the country’s strategic security "would be impossible" without this.
Initially there was no enthusiasm in reaction of the Western Polish allies to this proposal. But the head of the Polish government-in-exile Władysław Sikorski did not give up and prepared during 1940−42 four memorandums of post-war arrangement of Eastern Europe. In those documents East Prussia was considered a part of the Polish state.

Władysław Eugeniusz Sikorski
In 1939–43 the Prime Minister of the Polish government-in-exile
Initially there was no enthusiasm in reaction of the Western Polish allies to this proposal. But the head of the Polish government-in-exile Władysław Sikorski did not give up and prepared during 1940−42 four memorandums of post-war arrangement of Eastern Europe. In those documents East Prussia was considered a part of the Polish state.
The signing of an agreement on the restoration of diplomatic relations between the USSR and the Polish Republic government-in-exile on July 30, 1941.
The signing of an agreement on the restoration of diplomatic relations between the USSR and the Polish Republic government-in-exile on July 30, 1941.
Moscow received information about Polish claims to the Prussian territories. At first, the Soviet authorities did not see anything seditious in this, hoping thereby after the end of the war to compensate Poland for the loss of Western Ukraine and Western Belarus. But it should be kept in mind that relations between the Polish government-in-exile and Moscow deteriorated, so it is not surprising that the Soviet Union also turned its gaze to a part of East Prussia.
For the first time, the wish to annex a part of Prussia to the Soviet Union was discussed during the negotiations by Joseph V. Stalin and Vyacheslav M. Molotov with the British Foreign Secretary Anthony Eden who arrived in Moscow in December 1941. One of the drafts of the protocol, which was not finally agreed upon, contained the following proposal: after the war the southern part of East Prussia would become a part of Poland, whereas the rest of this German province, "adjacent to Lithuania", including Königsberg, would go to the Soviet Union for a period of 20 years as a guarantee of reparation of loss from the war against Germany.

Anthony Eden
In 1940–45 the British Foreign Secretary
For the first time, the wish to annex a part of Prussia to the Soviet Union was discussed during the negotiations by Joseph V. Stalin and Vyacheslav M. Molotov with the British Foreign Secretary Anthony Eden who arrived in Moscow in December 1941. One of the drafts of the protocol, which was not finally agreed upon, contained the following proposal: after the war the southern part of East Prussia would become a part of Poland, whereas the rest of this German province, "adjacent to Lithuania", including Königsberg, would go to the Soviet Union for a period of 20 years as a guarantee of reparation of loss from the war against Germany.
Russian historian Alexander V. Zolov believes that "apparently, this is where the current myth about the temporary nature of the Kaliningrad region's stay in the Russian Federation comes from".
Vyacheslav M. Molotov meets Anthony Eden, who arrived in Moscow on December 7, 1941.
Vyacheslav M. Molotov meets Anthony Eden, who arrived in Moscow on December 7, 1941.
After a series of defeats of Nazi Germany at Stalingrad and on the Kursk Bulge, the issue of the postwar arrangement of Europe turned from theoretical into practical field. Thus, in September 1943 within the People’s Commissariat for Foreign Affairs of the USSR the Commission on post-war order and preparation of peace treaties was established. Maxim M. Litvinov became the head of the Commission. Among other topics, the Commission discussed the future of the German-Soviet border.
Soon the issue of the partition of East Prussia was raised during the Tehran Conference (November 28 — December 1, 1943), in which Joseph V. Stalin, Winston Churchill and Franklin D. Roosevelt, the leaders of the allied powers, took part. During the discussion of the question of the western Polish border on November 30, 1943 Winston Churchill repeated twice that Russia deserved the access to warm water ports. Joseph V. Stalin supported the proposal of the British Prime Minister and noted that at the proper time that question could be discussed.

Maxim M. Litvinov
The Chairman of the Commission on post-war order and preparation of peace treaties
Soon the issue of the partition of East Prussia was raised during the Tehran Conference (November 28 — December 1, 1943), in which Joseph V. Stalin, Winston Churchill and Franklin D. Roosevelt, the leaders of the allied powers, took part. During the discussion of the question of the western Polish border on November 30, 1943 Winston Churchill repeated twice that Russia deserved the access to warm water ports. Joseph V. Stalin supported the proposal of the British Prime Minister and noted that at the proper time that question could be discussed.
The leaders of the countries of the anti-Hitler coalition during the Tehran Сonference on November 28, 1943.
The leaders of the countries of the anti-Hitler coalition during the Tehran Сonference on November 28, 1943.
The next day, on December 1, Winston Churchill proposed to finally approve the incorporation of the territory of East Prussia into Poland. In response to this proposal Joseph V. Stalin, unexpectedly for his allies, declared:
“
The Russians do not have ice-free ports on the Baltic Sea. Therefore, the Russians would need the ice-free ports of Königsberg and Memel and the corresponding part of the territory of East Prussia. Moreover, from historical point of view, these lands were primordially Slavic. If the British agree to transfer this territory to us, we will agree with the wording proposed by Churchill.
Joseph V. Stalin
In response, the British Prime Minister promised to study this "interesting proposal".
Russian historian Alexander V. Zolov writes: "There were no objections either from Churchill or from Roosevelt. For both leaders, Prussia was the seat of the trouble for Germany and the world, and the maximum possible weakening and reduction of its size seemed to be a necessary step in pacifying the defeated enemy. In addition, there was a huge relief caused by Stalin's promise to enter the war against Japan after the end of combat activity in Europe".
In a letter to Winston Churchill dated February 4, 1944 Joseph V. Stalin reiterated his claims to the northeastern part of East Prussia, including Königsberg as an ice-free port.

Personal and secret from Premier J. V. Stalin to the Prime Minister, Mr Winston Churchill:
“
As regards your statement to the Poles that Poland could considerably extend her frontiers in the West and North, we are in agreement with that with, as you are aware, one amendment. I mentioned the amendment to you and the President in Tehran.

We claim the transfer of the north-eastern part of East Prussia, including the port of Königsberg as an ice-free one, to the Soviet Union. It is the only German territory claimed by us. Unless this minimum claim of the Soviet Union is met, the Soviet Union's concession in recognising the Curzon Line becomes entirely pointless, as I told you in Tehran.
Joseph V. Stalin
In his reply, Winston Churchill informed Joseph V. Stalin that he first informed Poland about the "just" desire of the USSR to establish a border in East Prussia with the inclusion of Königsberg in the "Russian territory".

Received on February 27, 1944. Most secret and personal message from Mr Winston Churchill to Marshal Stalin:
“
The information came as a shock to the Polish Government, who see in such a decision substantial reduction in the size and in the economic importance of the German territory to be incorporated in Poland by way of compensation. But I stated that, in the opinion of His Majesty's Government, this was a rightful claim on the part of Russia. Regarding, as I do, this war against German aggression as all one and as a thirty-years' war from 1914 onwards, I reminded M. Mikolajczyk of the fact that the soil of this part of East Prussia was dyed with Russian blood expended freely in the common cause. Here the Russian armies advancing in August 1914 and winning the battle of Gumbinnen and other actions had with their forward

thrusts and with much injury to their mobilisation forced the Germans to recall two army corps from the advance on Paris which withdrawal was an essential part in the victory of the Marne. The disaster at Tannenberg did not in any way undo this great result. Therefore it seemed to me that the Russians had a historic and well-founded claim to this German territory.
Mr Winston Churchill
Russian historian Yury V. Kostyashov writes: "Thus, even before the end of the war, the issue of transferring East Prussia to Poland and the Soviet Union was agreed in principle among the allies in the anti-Hitler coalition".
By the way, Soviet politicians also considered the option for Königsberg and the area adjacent to it, including the Memel region, after the war to become a part of the Lithuanian SSR. This proposal was voiced by one of the members of the Litvinov Commission, Ambassador Extraordinary and Plenipotentiary of the USSR Yakov Z. Surits.
Russian historian Alexander V. Zolov mentions the following: "Apparently, there were quite a few people in Moscow who, for some reason, held affection for the new Soviet republic. However, common sense triumphed. At the insistence of the Soviet military, who considered Königsberg not just a port city, but an outpost allowing to control the southern Baltic, in the summer 1944 it was decided not to transfer it to Lithuania, but to leave the region to the Russian Federation".
Despite the British Prime Minister’s statement about the just Soviet claims to a part of East Prussia, Polish politicians-in-exile still hoped that the position of Joseph V. Stalin was not final and perhaps it would be still possible to convince him to abandon these demands. But the Soviet leader did not even think about it. In July 1944, an Agreement on the Soviet-Polish border was concluded between the government of the USSR and the pro-Soviet Polish Committee for National Liberation, one of the articles of which stated that "the northern part of East Prussia with the city and port of Königsberg goes to the Soviet Union, and the entire the rest of East Prussia, as well as the Danzig region with the city and port of Danzig, goes to Poland."
The results of the war were summed up at the Berlin (Potsdam) Conference of the leaders of the three powers, which was held from July 17 to August 2, 1945. During the conference, the United States and Great Britain confirmed their consent to the partition of East Prussia and the transfer of Königsberg with the adjacent area to the Soviet Union. The "Protocol of proceedings" of the Conference stated that the actual frontier of the territory for transfer was "…subject to expert examination". And then: "The President of the United States and the British Prime Minister have declared that they will support the proposal of the Conference at the forthcoming peace settlement".

However, as a result such a conference did not take place due to contradictions between the former allies and the "Cold War" that broke out soon.
Delegations of the Big Three at the negotiating table at the Potsdam Conference.
Delegations of the Big Three at the negotiating table at the Potsdam Conference.
Nevertheless, according to Yury V. Kostyashov, "in the international community, the legality of the Potsdam decisions had never been questioned and had been repeatedly reaffirmed in other international legal acts, including the Treaty between the FRG and the USSR (1970), Final Act of the Conference on Security and Cooperation in Europe (1975) and Treaty on the Final Settlement with respect to Germany signed in Moscow by the USSR, the USA, Great Britain, France, the German Democratic Republic and the Federal Republic of Germany, which secured the final character of the external borders of the united Germany".
In 2020, the amendments to the Constitution of the Russian Federation have been adopted on the inviolability of the borders of Russia and the impossibility of seizing Russian territories. In connection with the facts given, the claims of a number of politicians and public figures from the Baltic states, Poland and Ukraine about Russia’s ineligibility for the Kaliningrad region are unfounded.
References

  • Zolov, A. V. [2020]. "Russians need an ice-free port on the Baltic Sea" (Revisiting the history of transfer of Königsberg to the Soviet Union) / Zolov, A. V. "Russkim nuzhen nezamerzayushchij port na Baltike" (K voprosu ob istorii peredachi Kyonigsberga Sovetskomu Soyuzu). Available at: special.kantiana.ru/war#service
  • Kostyashov, Yu. V. [2018]. The formation of the Kaliningrad region / Kostyashov, Yu. V. Obrazovanie Kaliningradskoj oblasti. Available at: www.youtube.com/watch?v=h5DcZyRueeQ
  • Sal'kov, A. P. (2003). East Prussia in the plans of postwar arrangement of Europe. In The Questions of History, 12, p. 27−46. / Sal'kov, A. P. Vostochnaya Prussiya v planah poslevoennogo territorial'nogo pereustrojstva Evropy. In Voprosy istorii, 12, s. 27−46.
  • The Relations of the Soviet Union and England during the Great Patriotic War. (1983). Moscow. Vol. 1, 2. / Sovetsko-anglijskie otnosheniya vo vremya Velikoj Otechestvennoj vojny. Moskva. T. 1, 2.
  • USSR and the German Question. 1941−1949: Documents from the Foreign Policy Archives of the Russian Federation: in 2 Vols. Vol. 1: 22 June, 1941 — 8 March, 1945. (1996). Moscow. / SSSR i germanskij vopros. 1941−1949: Dokumenty iz Arhiva vneshnej politiki Rossijskoj Federacii: V 2-h t. T. 1: 22 iyunya 1941 g. — 8 maya 1945 g. Moskva.
  • The Soviet Union at the International Conferences during the Great Patriotic War of 1941−1945. (1984). Moscow. Vol. 2. / Sovetskij Soyuz na mezhdunarodnyh konferenciyah perioda Velikoj Otechestvennoj vojny 1941−1945 gg. Moskva. T. 2.