▁which ▁occupied ▁the ▁territory ▁during ▁World ▁War ▁I , ▁retre ated ▁from ▁the ▁area ▁in ▁May ▁ 1 9 1 9 , ▁they ▁turned ▁over ▁administration ▁to ▁the ▁Lith u ani ans . ▁Trying ▁to ▁prevent ▁an ▁armed ▁conflict ▁between ▁Poland ▁and ▁Lith u ania , ▁the ▁Ent ente ▁drew ▁a ▁dem arc ation ▁line , ▁known ▁as ▁the ▁F och ▁Line . ▁The ▁line ▁assigned ▁much ▁of ▁the ▁disput ed ▁Su w ał ki ▁( Su val ka i ) ▁Region ▁to ▁Poland ▁and ▁required ▁the ▁Lith uan ian ▁Army ▁to ▁retre at . ▁While ▁the ▁Lith u ani ans ▁retre ated ▁from ▁some ▁areas , ▁they ▁refused ▁to ▁leave ▁Se j ny ▁( Se ina i ), ▁because ▁of ▁its ▁major ▁Lith uan ian ▁population . ▁Polish ▁irregular ▁forces ▁began ▁the ▁u pr ising ▁on ▁August ▁ 2 3 , ▁ 1 9 1 9 , ▁and ▁soon ▁received ▁support ▁from ▁the ▁regular ▁Polish ▁Army . ▁After ▁several ▁military ▁sk irm ishes , ▁Polish ▁forces ▁secured ▁Se j ny ▁and ▁the ▁Lith u ani ans ▁retre ated ▁behind ▁the ▁F och ▁Line . ▁ ▁The ▁u pr ising ▁did ▁not ▁solve ▁the ▁larger ▁border ▁conflict ▁between ▁Poland ▁and ▁Lith u ania ▁over ▁the ▁eth n ically ▁mixed ▁Su w ał ki ▁Region . ▁Both ▁sides ▁compla ined ▁about ▁each ▁other ' s ▁repr ess ive ▁measures . ▁The ▁conflict ▁intens ified ▁in ▁ 1 9 2 0 , ▁causing ▁military ▁sk irm ishes ▁of ▁the ▁Polish – L ith uan ian