After the collapse of the Central Powers and the Armistice of 11 November 1918, Vladimir Lenin's Soviet Russia annulled the Treaty of Brest-Litovsk and moved forces westward to reclaim the Ober Ost regions abandoned by the Germans.

[30] The Paris Peace Conference (1919–1920) had not made a definitive ruling in regard to Poland's eastern border but on 8 December 1919, the Allied Supreme War Council issued a provisional boundary (its later version would be known as the Curzon Line).

[34] From late 1919, Lenin, encouraged by the Red Army's civil war victories over the White Russian forces and their Western allies, began to envision the future of world revolution with greater optimism.

[48][64] On 21 November 1919, after contentious deliberations, the Allied Supreme War Council mandated Polish control over eastern Galicia for 25 years, with guarantees of autonomy for the Ukrainian population.

[50][65] Jan Kowalewski, a polyglot and amateur cryptographer, broke the codes and ciphers of the army of the West Ukrainian People's Republic and of General Anton Denikin's White Russian forces.

[21][23] On 10 February, Soviet Russia's People's Commissar for Foreign Affairs Georgy Chicherin wrote to Polish Prime Minister Ignacy Paderewski, proposing resolution of matters of disagreement and establishment of relations between the two states.

[74] A Red Army assault between the Daugava and Berezina Rivers was repelled in October and the front had become relatively inactive with sporadic encounters only, as the line designated by Piłsudski to be the goal of the Polish operation in the north was reached.

[82][93] Early in the talks, Boerner informed Marchlewski that Poland had no intention of renewing its offensive; it allowed the Soviets to move forty-three thousand troops from the Polish front to fight Denikin.

[104] While the Soviet overtures generated considerable interests on the parts of the socialist, agrarian and nationalist political camps, the attempts of the Polish Sejm to prevent further warfare turned futile.

Piłsudski and his collaborators stressed what they saw as the increasing with time Polish military advantage over the Red Army and their belief that the state of war had created highly favorable conditions for Poland's economic development.

Preparations for a large-scale resumption of hostilities were being finalized and the newly declared (over the protest of a majority of parliamentary deputies) marshal and his circle expected the planned new offensive to lead to the fulfillment of Piłsudski's federalist ideas.

[108] To protest the alliance and the upcoming war over Ukraine, Stanisław Grabski resigned the chairmanship of the foreign affairs committee in the Sejm, where the National Democrats were a dominant force (their approval would be needed to finalize any future political settlement).

It ranged from activities approved and promoted at the highest level, such as the widespread theft of trains loaded with goods, to plunder perpetrated by Polish soldiers in Ukrainian countryside and cities.

In his 29 April and 1 May letters to General Kazimierz Sosnkowski and Prime Minister Leopold Skulski, Piłsudski emphasized that the railroad booty had been enormous, but he could not divulge further because the appropriations took place in violation of Poland's treaty with Ukraine.

[110][134] The "3rd Russian Army" reached over ten thousand battle-ready soldiers and in early October 1920 was dispatched to the front to fight on the Polish side; they did not engage in combat because of the armistice that took effect at that time.

[134] Soviet losses and the spontaneous enrollment of Polish volunteers allowed rough numerical parity between the two armies;[48] by the time of the Battle of Warsaw, the Poles may have had gained a slight advantage in numbers and logistics.

European arms merchants proceeded with supplying the Soviets with items needed by the military, for which the Russian government paid with gold and valuables taken from the imperial stock and confiscated from individuals.

[151] On 26 April, in his "Call to the People of Ukraine", Piłsudski told his intended audience that "the Polish Army would only stay as long as necessary until a legal Ukrainian government took control over its own territory".

[158] On 30 May 1920, General Aleksei Brusilov, the last Tsarist commander-in-chief, published in Pravda an appeal titled "To All Former Officers, Wherever They Might Be", in which he encouraged anti-Bolshevik Russians to forgive past grievances and join the Red Army.

[149] The Polish high command attempted to defend the Bug River line, reached by the Russians on 30 July, but the quick loss of the Brest Fortress forced a cancellation of Piłsudski's plans.

Stalin thwarted Sergey Kamenev's orders and directed the formations under Budyonny's command to close on Zamość and Lwów, the largest city in eastern Galicia and garrison of the Polish 6th Army.

[48][174] On 6 July, Piłsudski was outvoted in the council, which resulted in the trip of Prime Minister Grabski to the Spa Conference in Belgium made to request Allied assistance for Poland and their mediation in setting up peace negotiations with Soviet Russia.

[226] The Soviet advance toward Warsaw and Modlin was halted and had soon turned into a retreat, which contributed to the success of the drive by the main Polish formation coming from the Wieprz River area under Piłsudski's command.

On my shoulders, as Commander-in-Chief, and in your hands, as defenders of the Fatherland, the nation has placed the heavy task of securing Poland's existence, gaining for it respect and importance in the world, and giving it full independent control of its destiny.

[241] The Soviets, in addition to their battlefield losses, were pressured by events that necessitated the use of their military elsewhere, such as developments in the Turkish–Armenian War, Pyotr Wrangel's White Army still occupying the Crimea, or peasant rebellions in Russia.

[244][252] In the east, Polish culture was weakly represented even in the cities, except for a few in the western part of the disputed territories, and Grabski refrained from seeking a border along the so-called Dmowski's Line, previously promoted by his movement.

[226] The "Grabski Corridor", a strip of land inserted to separate Lithuania from Russia and connect Poland to Latvia, made Piłsudski's so-called Żeligowski's Mutiny and the Polish annexation of the Vilnius area possible.

However, Polish military activities, especially the so-called Żeligowski's Mutiny launched two days after the Suwałki Agreement, allowed Poland to capture the Vilnius Region, where a Polish-dominated Governance Committee of Central Lithuania was formed.

Internally, the newly reestablished Polish state had proved its viability, as an overwhelming majority of its people contributed to the defense of the country and turned out insensitive to Bolshevik appeals for joining the revolution.

[116][277][278] Andrzej Chwalba lists a number of ways in which the Polish military victory in reality turned out being a loss (the fundamental status quo – Poland's sovereign existence – had been preserved).