Complex sentence: Soviet Boundary and Friendship Treaty transferred Lithuania to the Soviet sphere of influence, which would serve as the justification that enabled the Soviet Union to occupy Lithuania on June 15,1940 and to establish the Lithuanian SSR.
Split into simpler sentences: Soviet Boundary and Friendship Treaty transferred Lithuania to the Soviet sphere of influence. This transfer would be the justification that enabled the Soviet Union to occupy Lithuania on June 15,1940 and to establish the Lithuanian SSR.