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

Here is a concise summary for each country you listed — which territories they lost and which (if any) they gained in Europe during or immediately after World War II.

1) Czechoslovakia
- Lost territory: Carpathian Ruthenia (Subcarpathian Rus') — annexed by the Soviet Union in 1945 and incorporated into the Ukrainian SSR.
- Gained territory: none permanently new from other countries after WWII (the Sudetenland and other areas taken by Germany were simply restored to Czechoslovakia; some short-lived Polish annexations of 1938–39 were reversed).

2) Finland
- Lost territory: substantial areas ceded to the Soviet Union under the Moscow Peace Treaty (1940) and the 1944 Armistice — chiefly the Karelian Isthmus (including Viipuri/Vyborg), parts of Salla, and the Petsamo region (and Finland had to lease Porkkala naval base to the USSR until 1956).
- Gained territory: none (Finland did not acquire new permanent territory).

3) Romania
- Lost territory: Bessarabia and northern Bukovina (and the Hertsa region) to the Soviet Union in 1940; these losses were confirmed after the war (Moldavian SSR and parts to the Ukrainian SSR). Southern Dobruja had been returned to Bulgaria in 1940 and remained Bulgarian after the war.
- Gained territory: Northern Transylvania — returned to Romania from Hungary in 1944 and confirmed by the 1947 Paris Peace Treaty (reversing the 1940 Second Vienna Award).

4) Lithuania
- Lost territory: as an independent state Lithuania was occupied and annexed by the Soviet Union (1940; re‑established as Lithuanian SSR after 1944), i.e. it lost state sovereignty and became part of the USSR.
- Gained territory: the Klaipėda/Memel region (returned from Germany and incorporated into the Lithuanian SSR after 1945). (Vilnius region, transferred to Lithuania by the Soviets in 1939, remained part of the Lithuanian SSR.)

5) Poland
- Lost territory: the eastern borderlands (the Kresy: large areas east of the Curzon Line — e.g. Lwów/Lviv, Wilno/Vilnius areas) were annexed by the Soviet Union (incorporated into the Ukrainian and Byelorussian SSRs).
- Gained territory: large former German territories east of the Oder–Neisse line (the so-called "Recovered Territories"): much of Silesia, Pomerania, the former Free City of Danzig/Gdańsk area, and southern parts of East Prussia (the Polish state was shifted westward).

6) Soviet Union
- Lost territory: none of significance in Europe after the war (it consolidated wartime gains). Note that the USSR absorbed several countries and regions rather than losing them.
- Gained territory: substantial European gains — the three Baltic states (Estonia, Latvia, Lithuania, annexed 1940 and re‑annexed after 1944), the eastern parts of pre‑war Poland (the Kresy), Bessarabia and northern Bukovina from Romania (forming the Moldavian SSR and additions to the Ukrainian SSR), parts of Finland (Karelian areas, Petsamo), and northern East Prussia including Königsberg/Kaliningrad.

If you want, I can produce a simple map-like listing (region → new owner) or give dates and treaty names for each change.