Question

Which nations lost territory and which gained territory in Europe during or immediately after the Second World War? Some may have lost territory in one region and gained land in another.

lost territory:
gained new territory:

1. Czechoslovakia
2. Finland
3. Romania
4. Lithuania
5. Poland
6. Soviet Union

Answer 1

Below I list, for each country you named, the main territory losses and gains in Europe during or immediately after WWII (rough time-frame: 1939–1947). Short notes on dates/agreements follow each item.

1) Czechoslovakia
- Lost territory:
- Carpathian (Subcarpathian) Ruthenia (transferred to the USSR in 1945 and incorporated into the Ukrainian SSR).
- (Earlier, in 1938–39 it had lost the Sudetenland and other areas to Germany/Hungary, but most of those were restored after the war.)
- Gained territory:
- None of consequence as a result of the post‑war settlements — Czechoslovakia was largely restored to its 1937 borders except for the loss of Ruthenia.

2) Finland
- Lost territory:
- Large parts of Finnish Karelia (including Viipuri/Vyborg), parts of the Karelian Isthmus, Salla, and the Petsamo area — ceded to the USSR in the 1940 Moscow Peace Treaty and confirmed/extended by the 1944 armistice and 1947 Paris Peace Treaty.
- Gained territory:
- None permanently. (Finland temporarily occupied some Soviet territory during the Continuation War, but these were returned under the 1944 armistice.)

3) Romania
- Lost territory:
- Bessarabia and Northern Bukovina (and the Hertsa region) to the Soviet Union in June 1940; these losses were confirmed after 1944 and in the 1947 Paris Peace Treaty.
- (Also lost Northern Transylvania to Hungary in the 1940 Second Vienna Award, but that was reversed after the war.)
- Gained territory:
- Northern Transylvania was restored to Romania after the Soviet advance and confirmed in the 1947 Paris Peace Treaty.

4) Lithuania
- Lost territory / status:
- Lost independence and was annexed by the Soviet Union in 1940 (became the Lithuanian SSR); as an independent state Lithuania ceased to exist de facto until 1990.
- Gained territory:
- The Klaipėda (Memel) region (which had been taken by Germany in 1939) ended up incorporated into the Lithuanian SSR after the war. (Overall, the Lithuanian SSR’s borders after WWII differed from the interwar independent Lithuanian state, but the dominant change was loss of independence/absorption into the USSR.)

5) Poland
- Lost territory:
- Eastern borderlands (the Kresy: large parts of eastern Poland, including Lwów/Lviv, Wilno/Vilnius area in practice) were annexed by the Soviet Union (incorporated into the Ukrainian and Belarusian SSRs) — formalized by post‑war arrangements (Potsdam / Soviet decisions).
- Gained territory:
- Large former German territories west and north of the new Polish line (the so‑called "Recovered Territories"): Silesia, most of Pomerania, the southern part of East Prussia and other areas east of the Oder–Neisse line — placed under Polish administration after Potsdam and confirmed by post‑war treaties.

6) Soviet Union
- Lost territory:
- No significant territorial losses in Europe as a result of WWII; the USSR was a net territorial gainer in Europe.
- Gained territory:
- Baltic states (Estonia, Latvia, Lithuania) — annexed in 1940 and again after 1944.
- Eastern Poland (the Kresy) — annexed in 1939–1940 and confirmed after the war.
- Bessarabia and Northern Bukovina from Romania (1940; reasserted after 1944).
- Carpathian Ruthenia from Czechoslovakia (1945).
- Parts of Finland (Karelia, Petsamo) ceded after 1940 and confirmed 1944–47.
- Northern part of East Prussia (including Königsberg/Kaliningrad) — taken from Germany at the end of the war and retained by the USSR.

If you want, I can produce a simple timeline (by year and treaty) showing when each of those losses/gains happened and which treaties or conferences formalized them.