Why did Poland's borders move west after World War II?

At Yalta and Potsdam in 1945, the Allies let the Soviet Union keep eastern Poland, roughly up to the Curzon Line, and compensated Poland with German lands east of the Oder and Neisse rivers. The country as a whole shifted west.

Which cities did Poland lose and gain in 1945?

Poland lost Lwów, now Lviv in Ukraine, and Wilno, now Vilnius in Lithuania. It gained German cities including Breslau, now Wrocław, and Stettin, now Szczecin, as well as Danzig, now Gdańsk.

Did Poland get smaller after World War II?

Yes. It lost roughly 180,000 square kilometres in the east and gained roughly 100,000 in the west, ending up about a fifth smaller than before the war.

When did Germany accept the Oder-Neisse border?

West Germany recognised it in the 1970 Treaty of Warsaw, and reunited Germany confirmed it in the German-Polish Border Treaty of November 1990.

Why Poland moved west.

In 1945 Poland did something countries almost never do: it moved. It lost a huge slice of land in the east, gained German land in the west, and ended up sitting about 200 kilometres further west than before the war.

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The Poland of 1939

Between the two world wars, Poland stretched much further east than it does today. Its eastern lands, which Poles call the Kresy, included the great city of Lwów and the city of Wilno, home to large Ukrainian, Belarusian, Jewish and Lithuanian communities as well as Poles.

In August 1939 Nazi Germany and the Soviet Union signed a pact with a secret protocol that divided Poland between them. Germany invaded on 1 September, the Soviet Union on 17 September.

Stalin keeps the east

When the war turned, Stalin made it clear he would keep eastern Poland. At the Tehran Conference in 1943 and at Yalta in February 1945, Britain and the United States accepted that Poland's new eastern border would run roughly along the Curzon Line, a line first proposed in 1919 and 1920 and named after the British foreign secretary Lord Curzon.

Poland would be "compensated" in the west, at Germany's expense.

Germany pays in the west

At the Potsdam Conference in the summer of 1945, the German lands east of the Oder and Neisse rivers were placed under Polish administration: Silesia, most of Pomerania, the southern part of East Prussia, and the former Free City of Danzig. German cities got Polish names. Breslau became Wrocław, Stettin became Szczecin, Danzig became Gdańsk.

In rough numbers, Poland lost about 180,000 square kilometres in the east and gained about 100,000 in the west. The country ended up roughly a fifth smaller, and in a different place.

The people moved too

Borders were only half of it. Millions of Germans fled or were expelled from the new western lands. Millions of Poles were moved out of the east, many of them resettled in the empty German towns. Whole institutions moved with them: the Ossolineum, a famous Polish library and publishing house founded in Lwów, was re-established in Wrocław.

Making it official

For decades the new western border was a political question. West Germany recognised the Oder-Neisse line in the 1970 Treaty of Warsaw. After reunification, Germany and Poland confirmed it for good in a border treaty signed in November 1990.

1939 Germany and the Soviet Union divide Poland
1943 Tehran: the Curzon Line is discussed
February 1945 Yalta: the east goes to the USSR
Summer 1945 Potsdam: Oder-Neisse lands to Poland
1970 West Germany recognises the border
1990 Reunited Germany confirms it

Sources

The Yalta Conference, 1945, US Department of State, Office of the Historian
The Potsdam Conference, 1945, Office of the Historian
Territorial changes of Poland immediately after World War II, Wikipedia
Curzon Line, Wikipedia
Oder–Neisse line, Wikipedia