20 years of independence, not 10-15 at most, & they'd had growing independence movements (especially in Estonia IIRC) for decades before that.
What was stolen from them? Many killed, mass deportations to the Gulag & exile i remote corners of the USSR, state-directed immigration of Russians which was well on the way to turning Latvians & Estonians into minorities in their own countries . . . the USSR was trying to turn them into peoples like the Mari or Komi or Mordovians, swamped by Russians & becoming more & more Russianised.
Klaipeda is atypical. It was mostly German (or Germanised - Lithuanians who'd lost their identity) by WW1, after being German-ruled for 500 years. BTW, the rest of Lithuania wasn't quite part of Poland. Poland & Lithuania united in the 14th century when the ruling prince of Lithuania (then pagan, the largest country in Europe, & expanding) was bribed to become a Catholic. The bribe consisted of a young Polish princess - & Poland.
The united country was called Poland-Lithuania for a few hundred years. Lithuania had a lot of history as an independent country.