The soldiers who remained in the Sarny Fortified Area were ordered in mid-September 1939 to abandon the bunkers and to move with their equipment towards the Romanian Bridgehead. On 16 September 1939, the eve of Soviet invasion of Poland, there were smaller units defending the area: two fortress battalions (Sarny) and (Malynsk), two border battalions (Rokitno) and (Berezne) and a cavalry squadron (Bystrzyce). There were altogether some 4000 soldiers, but they lacked heavy equipment, as it had been sent towards the German border in the summer of 1939.
Defence
On the morning of 17 September 1939 Soviet aircraft bombed the trucks, which were getting ready to move south. Also, the railway junction in Sarny was bombed, but the armoured train Marszalek repelled the enemy. However, Colonel Nikodem Sulik and General Wilhelm Orlik-Rueckemann decided to unload the equipment and to take up defence positions. The area was attacked by the Soviet 60th Rifle Division, whose advantage was crushing. The Poles defended the fortifications for three days. On 20 September, facing encirclement, they had to withdraw. However, soldiers in some bunkers did not get the order to withdraw, and in some places, the Poles resisted until 25 September. Several bunkers were blown up by the Red Army engineers with their crews, unknown number of soldiers were murdered, including seven officers, who were shot near the Orthodox church in the village of Tynne.
Among those killed was Lieutenant Jan Bolbot, who was in 1989 posthumously awarded the Virtuti Militari, Poland's highest military decoration. He commanded a platoon of 50 men who holed up in their bunker and
refused to surrender despite hopeless odds. Bolbot's men stopped Soviet attacks with heavy losses. Unable to beat the Poles, the Soviets set the bunker on fire. Bolbot and his entire command died in the flames.
In late September, the defenders of Sarny, as part of Independent Operational Group Polesie, took part in two major battles against the Red Army: the Battle of Szack and the Battle of Wytyczno.