Complex sentence: The Lithuanian government was not eager to cooperate with Poland, neither willing to cede disputed border areas-which they saw as part of the ethnographic Lithuania-and perceiving that it would lose its sovereignty under another Polish proposal, the proposed Międzymorze federation scheme.
Split into simpler sentences: The Lithuanian government was not eager to cooperate with Poland, nor was it willing to cede disputed border areas-which they saw as part of ethnographic Lithuania. The Lithuanians also believed they would lose their sovereignty under another Polish proposal, the proposed Międzymorze federation scheme.