In Moscow on September 28, 1939, officials from Nazi Germany and the Soviet Union turned a military conquest into a line on paper. Warsaw had capitulated the day before. Poland’s government had fled, but the Polish state had not consented to disappear. The treaty signed that night did not end the violence: it organized two occupations.
Part of the September 28 Today in History edition. Explore more in World War II.
A capital falls while diplomats meet
By the final week of September, the campaign begun by Germany on September 1 and widened by the Soviet invasion on September 17 had broken Poland’s conventional defense. German artillery and aircraft had battered Warsaw; its defenders agreed to surrender on September 27. Fighting elsewhere continued, and the Polish government rejected the claim that its republic had ceased to exist. In Moscow, however, Joachim von Ribbentrop returned to negotiate with Soviet foreign commissar Vyacheslav Molotov under Joseph Stalin’s watch.
The setting is documented in official photographs: delegations seated beneath chandeliers, maps opened on a table, signatures placed beside state seals. Those images can look orderly. Beyond them lay refugees, prisoners, destroyed towns and the early machinery of occupation. The distinction matters. This was diplomacy conducted after two armies had invaded the same country, not a settlement negotiated with Poland.
From the August pact to the September boundary
The negotiations rested on the German–Soviet Non-Aggression Pact of August 23, usually called the Molotov–Ribbentrop Pact. Its published clauses promised neutrality; a secret protocol assigned Finland, Estonia, Latvia, parts of Poland and Bessarabia to Soviet or German spheres. The agreement freed Adolf Hitler to attack Poland without expecting Soviet military resistance and allowed Stalin to pursue territorial security and expansion on the Soviet Union’s western border.
Events quickly outran the first sketch. German forces advanced farther east than the secret line, while Soviet forces entered eastern Poland from September 17. German and Soviet officers then arranged withdrawals, transfers and local handovers. Their encounter at Brest-Litovsk on September 22 made the collaboration visible. The September 28 treaty supplied the revised political framework.
What the treaty said—and concealed
The published German–Soviet Boundary and Friendship Treaty declared that the two governments had established a boundary between their “respective state interests” in the territory of the former Polish state. It described the line as a secure foundation for friendly relations and promised to suppress Polish “agitation” affecting the other power’s territory. The language transformed conquest into administrative vocabulary.
Secret supplementary protocols were more revealing. The spheres were revised: most of Lithuania moved into the Soviet sphere, while Germany received territory between the Vistula and Bug that had fallen within the earlier Soviet allocation. A map signed by Stalin and Ribbentrop traced the new division. Another protocol committed each regime to prevent Polish resistance from operating across the line. Economic arrangements deepened the temporary partnership, with Soviet raw materials helping Germany withstand blockade while German goods and technology moved east.
Two occupations, different systems of terror
The line did not create safety on either side. German occupation brought mass shootings, forced displacement, the destruction of Polish institutions and the escalating persecution of Jews that developed into genocide. Soviet rule brought arrests, deportations, executions, confiscation and an imposed political order. In 1940 the Soviet security service murdered thousands of Polish officers and other prisoners in the Katyn massacres. These crimes had distinct perpetrators and ideologies; the treaty made their territorial coexistence possible for twenty-one months.
Germany annexed western districts and placed central territory under the General Government. The Soviet Union incorporated eastern Polish lands into the Ukrainian and Byelorussian Soviet republics after staged elections. Both regimes presented territorial changes as lawful or popular. Polish underground structures and a government-in-exile preserved a contrary political fact: conquest had not erased sovereignty.
The partnership breaks
The treaty’s title spoke of “friendship,” but neither dictator had abandoned long-term hostility. Hitler’s racial and imperial program aimed at conquest in the east. Stalin used the interval to extend Soviet influence and rebuild strategic depth while continuing trade with Germany. On June 22, 1941, Operation Barbarossa ended the arrangement when German forces invaded the Soviet Union.
The boundary of 1939 vanished as a German–Soviet frontier, yet its consequences survived. Poland emerged after the war with borders shifted west: the Soviet Union retained most of the eastern territories it had occupied in 1939, while Poland received former German lands. Millions were displaced. The map signed in Moscow was not the final map of Europe, but it formed part of the chain of decisions that uprooted populations and remade borders.
Why September 28 matters
The date shows how aggressive war moves from battlefield to paperwork. Armies seized the ground; lawyers, diplomats and cartographers gave the seizure a vocabulary and a boundary. The agreement also defeats the comforting idea that ideological enemies cannot cooperate. Nazi Germany and the Soviet Union remained profound antagonists, yet both found a temporary bargain useful enough to partition a neighboring state.
Reading the treaty beside the record of occupation restores what its official prose excludes. Phrases such as “state interests” and “peace and order” conceal people whose homes, citizenship and lives were placed under regimes they had not chosen. The document is therefore both evidence and camouflage.
Diplomacy around the partition
Britain and France had declared war on Germany after the invasion of Poland, but the Soviet attack created a different diplomatic problem. The western Allies condemned Soviet action without opening a second war. Stalin meanwhile pressed the Baltic states into “mutual assistance” agreements that placed Soviet forces on their territory. In November, the Soviet Union attacked Finland, beginning the Winter War; the League of Nations expelled the Soviet Union in December. The September treaty belonged to a wider expansion of Soviet power under the cover created by the temporary arrangement with Germany.
Berlin also benefited immediately from the economic relationship. The British blockade made access to Soviet oil, grain and other raw materials strategically valuable. German machinery and manufactured goods moved in the opposite direction. These exchanges did not erase suspicion. They allowed both regimes to postpone conflict while pursuing nearer objectives.
A boundary through communities
Cartographic lines appear precise at diplomatic scale. On the ground, the boundary cut through provinces shaped by Polish, Jewish, Ukrainian, Belarusian, Lithuanian and other communities. Families fled in both directions, sometimes seeking to escape one occupier only to encounter the other. The two governments organized population transfers, including the movement of ethnic Germans out of Soviet-controlled areas. Confiscation and administrative upheaval changed property, schooling, employment and citizenship long before later wartime borders were settled.
Occupation also targeted leadership. German authorities arrested and murdered members of Poland’s political, religious and intellectual elites. Soviet authorities deported hundreds of thousands and imprisoned military officers, police, officials and civilians. The separate systems shared an interest in preventing an independent Polish public life from re-forming. That shared interest gives the treaty’s clause on suppressing Polish agitation its most threatening meaning.
The secret record and historical accountability
After Germany attacked the Soviet Union, Moscow joined the Allied coalition and the 1939 partnership became politically inconvenient. Soviet authorities denied the authenticity or significance of the secret protocols for decades even though captured German records made their substance known in the West. In December 1989, the Congress of People’s Deputies of the Soviet Union condemned the secret protocols. The documentary history matters because secrecy was not an incidental feature: it allowed the public promise of non-aggression to coexist with private plans for neighboring states.
Historians therefore distinguish among several layers of evidence—the published treaty, secret protocols, maps, military orders, occupation records and testimony from the people subjected to both regimes. No one layer is sufficient. The official document proves agreement; the occupation record shows what the agreement enabled.
Polish resistance challenged the premise that the conquerors had created a settled order. The underground state developed clandestine education, courts and political structures; armed organizations eventually united substantially within the Home Army. Exile authorities continued diplomatic work from abroad. Their existence is a reminder that recognition among aggressors did not make the partition legitimate.
The treaty also complicates later wartime memory. Germany’s genocidal war and invasion of the Soviet Union in 1941 made the Soviet Union essential to defeating Nazi Germany. That later role does not erase Soviet participation in the destruction of Poland in 1939. Historical sequence can hold both facts: a regime may be a co-aggressor in one phase and a decisive member of an anti-Nazi coalition in another.
For that reason, precise dates and terms matter. “Molotov–Ribbentrop Pact” usually names the August non-aggression agreement and its secret protocol; the September 28 Boundary and Friendship Treaty revised the territorial bargain after invasion. Collapsing them into one document hides how policy changed as armies moved. The second treaty was not a footnote. It was the adjustment that made the actual partition match the conquerors’ new decisions.
