To take and protect territory that was inhabited by Poles in majority. Or Belarussians.Who both got there as a result of steady peaceful acculturation. Ethnic Lithuanians were by then a small minority in Vilnius. You forget that not ALL Lithuanians spoke Lithuanian and not all wanted inndependent Lithuania. Many favoured the return to the "Rzeczpospolita" variant.In the East the conflict has started in April 1919, when Polish forces attacked Lithuania and USSR.
The only alternative was to cede those lands to the Soviets. If not for 1919-1921 there would not be even a tiny indepeneden Lithuania. But another Sovier SSR. And goodbye "Lithuanian national movement". You should be thankful for this "roberry".
Anywy. I heard that talk 1000s times. Its typical of Lithuanian nationalists who just cant come to terms with the fact that parts of Lithuania and Lithuanian elites became polonised or rutenised at the times of "Rzeczpospolita". But you also forget that this was a peaceful process and all that happened afterwards draws its root from there.